SKUIXS OF CYNODOXT REPTILES. ' 905 



The a-lisphenoid bone is one of the most interesting bones in 

 tlie skull, it is a lnrge flat bone which extends fi-om the pterygoid 

 below to the parietal above. It is well seen in the type skull of 

 C'ynognathv.s crateronoliis, but even better in the Capetown 

 specimen of C herryi (PI. XL VI. fig. 1). The upper portion of 

 the bone is irregularly quadrilateral. The uj^per side aiticulates 

 with the parietal, and the posterior with what I believe to be the 

 prootic. Between the alisphenoid and the prootic are two large 

 oval foramina. At the posterior and lower corner the alisphenoid 

 is continued as a slender bone to the quadrate. At its anterior 

 and lower corner it meets the pterygoids and clasps the Vjasi- 

 sphenoid. There appears to be an opening into the brain-cavity 

 between the base of the alisphenoid and the basisphenoid. 



The basisphenoid is clasped by the alisphenoids and meets the 

 basioccipital posterioi-ly. 



The basioccipital is a. small bone lying behind the basisphenoid. 

 It forms the middle part of the occipital condylar region. On 

 each side there is a large round foramen which is pretty certainly 

 the foramen for the exit of nerves ix., x., xi., and xii. 



Between the outer part of the basioccipital and the quadi'ate 

 there stretches a rounded pillow-like bone concerning which there 

 may be some difference of opinion. I believe it to be the 

 stapes, for reasons which will be stated later. 



The lower jaw is remarkable for the great size of the dentary, 

 which posteriorly nearly reaches the articulation. Elsewhere I 

 have dealt at some length with the structure of the jaw. The 

 splenial is long and slender. The surangular and angular are also 

 feeble splint-like Ijones. The articular is fairly well develojied but 

 short. I cannot satisfy myself that there is a distinct coronoid 

 bone as is stated by 8eeley. 



The only points in which the C'ynognathus skull is nearer to 

 the mammal than that of Bcxuria and Nythosaurus are : {a) the 

 closer approach of the jngal to the articulation, (?;) the greater 

 development of the dentary, (c) the greater reduction of the 

 angular and surangular, and {d) the more mammal-like occipital 

 condyle. On the whole it is not so near the mammalian ancestor 

 as either Bauria or Nythosaurus. 



The dental formula aj^pears to be i. „-, c. , m. ^. 



Trirachodon. 

 (Text-figs. 173 & 174.) 



Trirachodon is best known by the type skull which is in the 

 Albany Museum. Though the skull is immature and much 

 cruslied it is practically perfect. Two or three other known skulls 

 though imperfect show the uncrushed condition of the greater part 

 of the adult skull. 



The premaxilla is smaller than in Cynognathus, not meeting the 

 nasal behind the nostril, at least not on the face. 



