Osteostracans are the closest jawless relatives of jawed vertebrates, informing the gradual assembly of the vertebrate mineralised skeleton. Conflicting interpretations of their dermal skeletal histology arise from failure to account for topological variation, obscuring their significance in elucidating vertebrate skeletal evolution. To resolve this, we characterize the cranial and trunk dermal skeleton of a single individual of Tremataspis mammillata (Osteostraci, Thyestiida) at submicron resolution using synchrotron‐ and computed‐ tomography. Our results show that the architecture of the Tremataspis dermal skeleton is, for the most part, conserved over the skeleton and is broadly consistent with previous histological hypotheses based on 2D thin section study. We resolve debate over the homology of the basal layer, identifying it as osteogenic acellular isopedin rather than odontogenic elasmodine or metaplastic ossification of the stratum compactum of the dermis. We find topological variation between all dermal skeletal elements studied, and particularly between the cranial and postcranial dermal skeleton. This variation can be largely explained by reduction in differentiation due to geometric constraints imposed within smaller skeletal elements, such as scales. Our description of the dermal skeleton of Tremataspis mammillata provides a foundation for interpreting data from cursory topological samples of dermal skeletal diversity obtained in other osteostracans. This reveals general aspects of histological structure that must be ancestral for osteostracans and, likely, ancestral jawed vertebrates. Finally, we draw the distinction between hypotheses and descriptions in palaeohistology.

SUMMARY1. The effects of vagal stimulation and applied acetylcholine were compared on the isolated sinus venosus preparation of the toad, Bufo marinus.2. The effects of applied acetylcholine and of low-frequency, or short bursts of high-frequency vagal stimulation were abolished by hyoscine.3. When intracellular recordings were made from muscle cells of the sinus venosus, it was found that applied acetylcholine caused bradyeardia and a cessation of the heart beat which was associated with membrane hyperpolarization and a reduction in the duration of the action potentials. Much of the effect of acetylcholine can be attributed to it causing an increase in potassium conductance, 9K.4. When slowing was produced by low-frequency vagal stimulation, only a small increase in maximum diastolic potential was detected. During vagal arrest the membrane potential settled to a potential positive of the control maximum diastolic potential.5. In the presence of barium, much of the bradyeardia associated with vagal stimulation persisted. Although the bradycardia produced by added acetylcholine also persisted in the presence of barium, the effects of acetylcholine that could be attributed to an increase in gK were abolished.6. Addition of caesium ions produced bradyeardia with membrane potential changes similar to those seen during vagal stimulation.7. The results are discussed in relation to the idea that neuronally released acetylcholine reduces inward current flow during diastole. In contrast applied acetylcholine as well as reducing inward current flow during diastole also increases outward current flow by increasing 9

Tree-kangaroos are a unique group of arboreal marsupials that evolved from terrestrial ancestors. The recent discovery of well-preserved specimens of extinct tree-kangaroo species (genus Bohra) within Pleistocene cave deposits of south-central Australia provides a unique opportunity to examine adaptive evolution of tree-kangaroos. Here, we provide the first detailed description of the functional anatomy of the forelimb, a central component of the locomotor complex, in the extant Dendrolagus lumholtzi, and compare its structure and function with representatives of other extant marsupial families. Several features were interpreted as adaptations for coping with a discontinuous, uneven and three-dimensional arboreal substrate through enhanced muscular strength and dexterity for propulsion, grasping and gripping with the forelimbs. The forelimb musculoskeletal anatomy of Dendrolagus differed from terrestrial kangaroos in the following principal ways: a stronger emphasis on the development of muscles groups responsible for adduction, grasping and gripping; the enlargement of muscles that retract the humerus; and modified shape of the scapula and bony articulations of the forelimb bones to allow improved mobility. Many of these attributes are convergent with other arboreal marsupials. Tree-kangaroos, however, still retain the characteristic bauplan of their terrestrial ancestors, particularly with regard to skeletal morphology, and the muscular anatomy of the forelimb highlights a basic conservatism within the group. In many instances, the skeletal remains of Bohra have similar features to Dendrolagus that suggest adaptations to an arboreal habit. Despite the irony of their retrieval from deposits of the 'Treeless' Plain, forelimb morphology clearly shows that the species of Bohra were well adapted to an arboreal habitat. WARBURTON 3

SUMMARY1. Intracellular recordings were made from pacemaker cells lying in the sino-atrial node of guinea-pigs.2. Low-frequency vagal stimulation slowed the rate of generation of pacemaker action potentials; high-frequency stimulation stopped the generation of action potentials.3. During vagal stimulation the rate of diastolic depolarization was reduced with the action potential otherwise unchanged; when the heart stopped the membrane potential of pacemaker cells settled to a value positive of the maximum diastolic potential.4. In contrast, added acetylcholine caused membrane hyperpolarization and shortened the duration of action potentials.5. The effects of both added acetylcholine and vagally released acetylcholine were abolished by hyoscine.6. It is suggested that neurally released acetylcholine acts to change the balance between inward and outward current flow during diastole by modifying the properties of existing voltage-dependent channels. In contrast added acetylcholine appears to activate a different set of receptors which increase the potassium conductance of pacemaker cells.

SUMMARY1. The effects of sympathetic nerve stimulation on the generation of pacemaker action potentials, recorded from the sino-atrial node of the guinea-pig, were determined by using intracellular recording techniques.2. Trains of stimuli applied to the right stellate ganglion led to an increase in heart rate after a delay of a few seconds. During the initial phase of the tachyeardia the rate of discharge of pacemaker action potentials increased and the rate of diastolic depolarization increased, but both the peak diastolic potential and the maximum rate of rise of the action potentials were reduced. During the later phase of the tachycardia the peak diastolic potential, the amplitude of the action potentials, the maximum rate of rise and the rate of repolarization of the action potentials were increased.3. When membrane potential recordings were made from sino-atrial node cells, in which beating had been abolished by adding the organic calcium antagonist nifedipine, sympathetic nerve stimulation initiated excitatory junction potentials (EJPs) which had time courses similar to those of the tachycardias recorded from beating preparations.4. Although both the tachyeardias produced by either sympathetic nerve stimulation or added noradrenaline were largely abolished by fl-adrenoceptor antagonists, the membrane potential changes recorded during the responses to sympathetic nerve stimulation or added noradrenaline were different. Bath-applied noradrenaline caused a tachycardia which was associated with an increase in the amplitudes of pacemaker action potentials, an increase in the peak diastolic potential and a shortening in the duration of pacemaker action potentials.5. The addition of agents which cause the accumulation of cyclic AMP in the cytoplasm of the cells produced a tachycardia which was associated with a similar sequence of changes in the membrane potentials to those produced by added noradrenaline; again the membrane potential changes produced by these agents differed from those produced by sympathetic nerve stimulation.6. The results are discussed in relation to the idea that neurally released noradrenaline activates a set of receptors which cause tachycardia by increasing