646 S. W. WILLISTON 



never (?). Lacrimals usually not entering nares. Teeth thecodont 

 (or protothecodont?), rarely attached to splenials or vomers 

 (prevomers). Neck oj greater length. Neural arches never stout. 

 Ribs single- or double-headed, the cervical ribs sometimes di- 

 lated distally. Vertebral spines rudimentary, of moderate length 

 or greatly elongated. Abdominal ribs present or absent. Two 

 or three sacral vertebrae; tail short or of considerable length. 

 Posterior coracoid bone sometimes unossified. Cleithrum well 

 developed, vestigial or absent. Usually, but not always, a pubo- 

 ischiadic vacuity in pelvis. Ungual phalanges rarely obtuse, 

 usually claw-like. Legs usually less stout, sometimes very much 

 elongate; ectepicondylar foramen rarely present. No dermal 

 ossifications. Presacral vertebrae from twenty-four to at least 

 twenty-seven in number. 



The only characters, it is seen, by which an unknown genus 

 may be referred with assurance to one or the other of these orders 

 are found in the perforation or non-perforation of the temporal 

 roof, and the neural arches of the presacral vertebrae. The stout 

 neural arches are, however, not peculiar to the Cotylosauria 

 since they occur in the Proganosauria. Nevertheless, the Thero- 

 morpha are, as a whole, a more advanced group of reptiles than 

 the Cotylosauria, as observed in the longer neck; longer, more 

 ambulatory and prehensile legs, more slender bones, and espe- 

 cially in the constant reduction of the cranial bones. No thero- 

 morph has all the cranial elements of Seymouria or Pantylus, 

 and it is probable that none has all the elements found in Cap- 

 torhinus or Labidosaurus even, perhaps the most advanced types 

 of American Cotylosauria. And it is precisely in the skull struc- 

 ture that we may hope eventually to reach the most accurate 

 comprehension of the interrelations of the two groups. 



Ordinarily the differences known to exist between the different 

 groups of the Cotylosauria — Diadectosauria, Pantylosauria, 

 Pareiasauria and Procolophonia — might be considered of ordinal 

 value if occurring among hving reptiles. The Captorhinidae 

 or Procolophonidae, for instance, have in the posterior cranial 

 and temporal region the postparietal, tabulare, squamosal and 



