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Examination of the cardiac anatomy of lizards of the family Varanidae revealed that much of the previous literature contained basic errors both in description and interpretation. These are corrected and terminology of various authors is standardized.The varanid ventricle is similar to that of most other lizards in not being elongated, in having the base at right angles to the longitudinal axis, and in having the vertical septum weakly developed. However, it is not a typical lacertilian heart in that it has a number of characteristic ophidian features such as a muscular cone surrounding the cavum latero-dorsale and a prominent twisting of the cava. It lacks a gubernaculum cordis and a cartilaginous rod in the septum aortico-pulmonale, both of which are present in many non-varanid lizards, but seldom occur in snakes. The location of the heart is more posterior in the body than is true of other lizards.During preliminary investigations on reptilian cardiac physiology, it became apparent that there exists no clear understanding of basic squamate heart anatomy requisite for interpretation of experimental results. Sources of confusion have been ( 1 ) failure to standardize terminology and inconsistent and incorrect usage of terms, (2) improper orientation of materials, slides and/or drawings being examined or presented up-side-down and misinterpreted accordingly, and ( 3 ) an uncritical following of the mistakes of previous authors. Webb ('69) presented a partial review of the literature, standardized terminology, and by re-examining hearts of the species upon which much of the anatomical literature is based, corrected many previous errors. A series of papers, of which this is the first, expands and revises that study. The present one treats the ventricle of the lizards of the family Varanidae. This family was selected first, because: (1) It belongs to a group (Platynota) claimed to be ancestral to the ophidian line (but see reviews by Bellairs and Underwood '51, McDowell and Bogert '54), so that an understanding of the anatomy of varanids J. MORPH., 134: 335-350. may be important for phylogenetic interpretations. ( 2 ) Much of the previous literature has been based on this family and correction of previous erroneous interpretations of varanid cardiac anatomy also clears up a number of problems related to the general structure of the squamate heart. ( 3 ) The varanid heart has been used as a "typical" squamate heart (see Literature below). This unwarranted assumption must be disposed of before proceeding with further comparative studies.The ventricle is emphasized because most of the confusion in the literature centers around this part of the heart. THE GENERALIZED SQUAMATE VENTRICLEThe ventricle is typically conical with the apex directed posteriorly. In most lacertilians, it lies in the midline of the body with the bulbar region facing ven- 335

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Amphibolurus inermis occurs throughout Australia except for the eastern third of the continent, the north—central coast, and the southwestern tip. It inhabits sandy soils. Body temperature rises in the morning, levels off and remains rather constant between 1000 and 1600 hr, and falls after 1600 hr. Thermoregulation involves changes in location and posture, color change, basking, burrowing, shade—seeking, and panting. The level of temperature maintained depends on geographic locality and immediate weather conditions and may depart widely from the temperature preferred in a laboratory gradient. The relative importance of specific behavioral responses varies with locality; burrowing and shade—seeking may be interchangeable. Amphibolurus inermis functions either as a thigmotherm or a heliotherm depending on local conditions and time of day. It is one of the most heat—resistant species known; the mean body temperature which causes loss of coordination is 48.5° C, and the lethal temperature is 49.3° C. Because of the extreme environments it occupies, A. inermis has narrow minimum thermal safety margins even though it has high temperature tolerances. Burrows tend to be located near thermoregulatory perches. When the sun—perch and the shade—perch are close together, an individual may have only one burrow; if the perches are further apart, a burrow will usually be located near each. If widely separated, auxiliary burrows may occur between the two types of perches. Burrows sometimes become too hot to serve in thermoregulation. Active individuals are found at all hours between sunrise and sunset in summer. However, individuals may seek shelter at midday. In winter the activity period is restricted to a few hours at midday. It is suggested that preferred temperatures may shift during the day, but that these levels cannot always be maintained. Head temperatures may be regulated more precisely than those of the body.

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