1888.] possibly Dual Oriyin of the Mammalia. 373 



met with general acceptance, and it is similarly supposed that all 

 other mammals must have followed the same route and must there- 

 fore also be descendants of some early reptile-like creature. 



The question, therefore, of this resemblance or non-resemblance of 

 the Ornithorhynchus teeth with any known reptilian teeth becomes a 

 question of much interest. The author of the recent communication, 

 Mr. Poulton, affirmed that the teeth were distinctly mammalian 

 teeth* 



I have long believed that no such teeth were to be found in any of 

 the Sauropsida, and the conviction I previously entertained has been 

 confirmed by a recent re-examination (ad hoc) of the dentition of 

 Reptiles extant and extinct, preserved in the National Collection ; and 

 I here desire to express my warm thaiiks to Mr. Gr. A. Boulenger and 

 Mr. Lydekker for the very kind and ready help I have received from 

 them. 



The results of my examination may be snmmed up as follows : — 

 The Sauropsidan tooth, from the lowest reptiles to Hesperornis, may 

 be described in general terms as a subcorneal structure in which 

 subsidiary additions or modifications may arise, which, however, never 

 cause it to resemble a mammalian molar — except, of course, such 

 exceptional mammalian molars as are themselves mere dental cones — 

 or to resemble the mandibular tooth described by Mr. Poulton as exist- 

 ing in Ornithorhynchus. That tooth was said to present the follow- 

 ing characters : — Towards its outer edge were two large cusps, one in 

 front of the other, and opposite to them were four or five very small 

 ones extending from behind forwards along the inner edge of the 

 tooth. The tooth above it was said to be conversely constructed, so 

 that the two in tei locked, the greater prominence of the upper tooth 

 being towards its internal edge. 



Nothing of this kind exists in any reptile. In reptiles the dental 

 cone may be laterally compressed and serrated at its margin, as in 

 Megalosaurus ; it may be less laterally comjDressed but serrated and 

 furnished with vertical prominences, as in Iguanodon. From this 

 we find transitions to the tricuspid tooth of Cyclura, and the summit 

 is subdivided into two or three cusps in a multitude of existing 

 lizards, while it may assume the form of a fleur-de-lys as in Ambly- 

 rhynchus. Very rarely (only in Teius and Dicrodon) there may be a 

 supplementary prominence on one side, which may attain to within a 

 short distance of the height of the main cone and thus present the 

 appearance of a single cone with a deep antero-posteriorly directed 

 groove. Finally, as in Empediasf there may be a central prominence 



* "The teeth probably represent some part of the molar series in the higher 

 mammals." 



f The Empedocles wolaris of Cope (see ' Amer. Phil. Soc. Proc.,' vol. 19, p. -17). 

 The specimens in our national collection are also thus labelled. 



2 e 2 



