54 1 U ERIC AN PERM I A N T 'ER TEBRA TES 



The presacral vertebrae, as stated, are all in close articulation; 

 the number is positively fixed at twenty-three. It is worthy of 

 note how common this number of presacral vertebrae seems to be 

 among the early land vertebrates. It is the number I concluded 

 was present in the temnospondyl Trematops, apparently the number 

 in Sauravus, Hylonomus, Eosauravus, and in Captorhinus. In 

 Limnoscelis there are, apparently, twenty-five, though this is not 

 certain. In Cacops, a temnospondyl, there are twenty-one, with 

 perhaps twenty-four in Eryops. Among the higher reptiles, Casea 

 has twenty-four, Varanosaiirus and Dimetrodon, twenty-seven. 



Nothing is more conspicuous in the skeleton of Seymouria than 

 the great development of the neural arches of the presacral verte- 

 brae, especially posteriorly; and the abrupt change from the broad 

 overroofing form of the presacrals to the narrow, more slender form 

 of the caudals. Not much can be seen of the atlas and axis, but, 

 from the third vertebra, the arches increase rapidly in transverse 

 expansion to the eighth or ninth; that is. throughout those bearing 

 the distally dilated ribs. With the ninth, the arches have acquired 

 nearly or quite their full width, remaining uniform to the sacrum. 

 On the first three or four vertebrae there are distinct, though small, 

 spines, perhaps six or eight millimeters in height ; from the fourth to 

 the sacrum the spines are mere tubercles. The structure of the 

 posterior vertebrae will be made sufficiently plain from the figures 

 given in Plate XIII, Figs. 5-8, from a vertebra found in the 

 Craddock bone-bed, and from an isolated specimen found on Coffee 

 Creek. The zygapophyses, it is seen, are very broad and flat. 

 The diapophyses are small protuberances rising high up from the 

 sides of the anterior zygapophyses; from the eighth vertebra to 

 the sacrum they do not project beyond the border of the zygapo- 

 physes; in front, however, they extend beyond them conspicuously, 

 their expanse as great as on the posterior vertebrae. The centra 

 are subcylindrical, with the lower border shorter, in some very 

 much shorter, than the length of the neural canal, allowing space 

 for the large intercentra. The intercentra are not as large propor- 

 tionally in the adult specimens as in the young, as figured and de- 

 scribed by me in Desmospondylus (Plate XXIX). Nevertheless 

 they are unusually large for a reptile in the adult even, so large 



