No. 434] THE AMERICAN PELYCOSAURIA. 



87 



(d) The increase in size of the ectopterygoid process of the 

 pterygoid bone. 



(e) Correlated with these changes the development of the 

 enormously elevated neural spines. 



Following is a short description of the skulls of the types 

 representing the advancing stages of the development. The 

 genera Dimetrodon, Embolophorus, and Naosaurus are not 

 described, as the first two have been very fully described (Baur 

 and Case '99 and Case 1 ) and Naosaurus is very similar to them 

 in the regions under discussion. 



Tkeropleura uniformis. — Only the portion of the skull ante- 

 rior to the posterior edge of the orbits is preserved ; but this 



made out. A careful examination of the skull compels me to 

 differ from the description by Cope in one or two points. He 

 mentions (Cope '80) the presence of "at least one large incisor 

 tooth," and says the incisor teeth are separated from the maxil- 

 lary teeth by a diastema. The anterior end of the snout is 

 crushed, but the incisor teeth are seemingly all preserved, and 

 I could not detect any one that was notably larger than the 

 others ; they are small and sharply recurved. Neither could I 

 detect the presence of a diastema marked by any concavity of the 

 alveolar edge of the maxillary bone such as is so persistent in 

 the other American Pelycosauria ; it is possible that there was a 

 lack of teeth at this point, but as the bone is somewhat crushed 

 at the point it is impossible to sav certainly. As mentioned by 

 Cope, there is no distinct maxillary canine, but the teeth increase 

 in size to near the middle of the series and then diminish so 

 that the 19 from the posterior end is the largest ; the whole 

 tooth line and the alveolar edge of the maxillary is straight 

 without a trace of the convexity downward that marks the more 

 specialized form. As described by Cope, the skull is low and 

 acuminate ; the orbits are large and the nares are near the ante- 



much is fairly free from the 



<, and the parts are easily 



rior end of the snout on the sides, 

 similar in general outline and app< 

 terosaurus, and in just the same cl 



