I n the neighbourhood of St. David's, the Arenig rocks, described by Mr. Hicks in his paper read at the last meeting of the Geological Society*, are the earliest in which Graptolites are known to occur; and yet, when once they appear, so diversified are their forms that these Pembrokeshire rocks are only equalled, in the number and variety of the genera they contain, by the Canadian Graptolite-bearing rocks of equivalent age known as the Quebec Group. In more ancient deposits two species only, belonging to one of the two great sections into which these fossils are divided, have hitherto been detected, viz. Dictyograptus (Dictyonema) socialis , Salter, and Dendrograptus Hallianus , Prout. The former occurring in the lower portion of the Tremadoc rocks of North Wales, and the latter in the equivalent strata (the Potsdam Sandstone of America), it is impossible to say which genus is the earlier, or whether the group is first represented in Britain or in America. Before the discovery, in 1872, of the extensive series of Graptolites which characterize the Lower Arenig rocks of Ramsey Island, the Skiddaw Slates of Cumberland were supposed to be our earliest Graptolite-bearing rocks; but it is now known that the lowest rocks of the Arenig Group exposed in the vicinity of St. David's, in which Graptolites abound, are of greater age than any part of the Skiddaw Slates yet described; and it is highly probable that they are also older even than the lowest beds of the Quebec Group known to contain
In this communication I purpose describing a few new forms of Graptolites which I have obtained at various times from the Llandeilo rocks of the south of Scotland. Of one species, first collected at Moffat in 1866, a brief diagnosis has previously been given, and the names of two others, from the lead-mining district of Lanarkshire, have already been published. One of these also occurs near Moffat. The remaining species are all from one or other of these richly fossiliferous districts. Of their position in the geological series I need only say here that the black, more or less carbonaceous, shale in which they occur, appears, from the fossils it contains, to correspond to the higher portion of the Llandeilo flags of Wales; that it is almost immediately succeeded by a series of beds (the Gala group) containing fossils of Caradoc or Bala age; and that the unfossiliferous flagstone, or greywacké, in which it occurs, reposes on rocks which have yielded to the persevering search of Prof. Elliot and Messrs. Lapworth and Wilson a few fossils of Cambrian age.
IX After the matter had been fully gone into, and the MSS. and drawings had been examined, Professor Huxley expressed his willingness to do what he could in the matter, but said he should require some fresh specimens of Ascidia mental// for dissection. These Mr. Norman undertook to procure for him, and this was done through the kind help of Mr. David Robertson, of Cumbrae, who went specially to Oban to obtain the specimens, which he sent to Huxley.* When a little more than four years had passed, the letter here given was received from Professor Huxley by Dr. Embleton. " 4, MAKLBOROUGH PLACE,, "LONDON, October 12th, 1879. "My DEAR SIR, " After my return from Newcastle, I forget how many years ago, 1 examined Albany Hancock's MSS. and drawings more carefully than I had before been able to do, and I confess that the work of making a presentable volume out of them did not appear to me to promise to be easy, but I was quite prepared to do my best. However, shortly afterwards, in talking over the matter with a Member of the Council of the Ray Society, he assured me that there was not the least * See ' The Naturalist of Cumbrae, being the Life of David Robertson ' (1891), by the Rev. T. E. R. Stebbiug, pp. 304-305. 106 22. A. inornata 108 23. A. Morel: branchial sac 127 24. A. inoUls 145 * Additions to the aiithors' MS. are placed within brackets in the usual manner, except obviously editorial footnotes. A few verbal corrections have been, made which are not so indicated. In the historical portion of the Introduction the original spelling-of generic and specific names has been retained. t This term is here retained in deference to the adoption of it by the authors, although the right of priority rests with Polyzoa. On this point, and also on the intimate relationship existing between the Tunicata and the Polyzoa, see Allnian's (Fresh-water Polyzoa ' (Ray Society, 1856), and the papers referred to on pages 6 and 43 of that work. 1 Z BRITISH TUNICATA. We shall, therefore, consider the Tunicata as the lowest members of the Molluscan series, touching, on the one hand, the Lamellibranchiata, on the other the Bryozoa and the Brachiopoda. They are all marine, shell-less, headless, footless mollnsks of a comparatively low organisation, having an elastic external envelope, test, or outer tunic, the homologue of the ordinary molluscan shell, and an internal tunic or mantle ; they are provided with a well-differentiated digestive apparatus, usually with a sac-shaped gill placed in front of the digestive organs, and leading to the mouth ; with rather well-developed muscular organs, in which the heart is tubular, forcing the blood for a while in one direction, and then reversing its action and forcing it in the opposite direction. They are all androgynous, with very complete reproductive organs, and all undergo a metamorphosis, while the greater * ['Historia Aninmlium/ lib. iv, cap. 6; and ' De Part. Anim.,' lib. iv, cap. 5.
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