The pattern of dermal-vertebral correlation in snakes and amphisbaenians

INTRODUCTION It has long been known that the arrangement of external scales retains a constant relation to the primary pattern of mesodermal segmentation. The ratio of the number of dermal scale rows or annuli to the number of vertebrae has, therefore, been considered to be of fundamental importance in squamate classification (Stehli, 1910; Camp, 1923). Yet the difficulty of determining it has fostered the presentation of hypotheses based on relatively few total counts; some such hypotheses have relied upon comparisons made by dissection along limited portions of the trunk. The general implication communicated by the many statements in the literature has been that the ratios are constant at the generic or familial level, that the ratios ordinarily represent simple, whole number relations (i.e. 1 : 1, 1 : 2, 2 : 1), and that the scale-vertebra relation is constant along the length of the body. Authors have disagreed regarding the presumed evolution of these regularities; thus Stehli (1910) argued that the 1 : 1 ratio was primitive, while Camp (1923) considered it to be most highly advanced. The most "primitive" snakes (Bellairs & Underwood, 1951) were generally stated to have a 2 : 1 and the "advanced" forms a 1 : 1 ratio. In contrast, the presumably most primitive amphisbaenid (Smalian, 1884) has generally been stated to have a 1 : 1 and all other, and presumably more advanced, forms, a 2 : 1 ratio. A recent report (Gans & Taub, 1965) showed a simple method of determining these ratios, emphasized that the ratios varied markedly at the species 1) Present address : Department of Biology, Canisius College, Buffalo, N.Y. 14208. level, and also showed that the relationship of scales to vertebrae within the Typhlopidae was not a simple, whole number ratio. This suggested the