TEURESTKIAL VERTEBRATA. 245 



articulation for a sino^le-headed lib. In the cervical re«:ion the articular surface for 

 the rib is continued downwards, forming a shallow groove on the posterior part of the 

 side of the intereentrum. This groove becomes shorter, and finally disappears from 

 the intercentra at some distance posterior to the ceivical region. In the cei'vical 

 region in some of the specimens a groove crosses the inferior side of the inter- 

 centrum, passing through and dividing into two parts the co.stal groove. It appears 

 to be a suture which cuts ott" a segment from the posterior side of tiie intereentrum. 

 This segment is cobssified with the inteiccntnini in most of the cervical vertebi-aa of 

 Ertjops viegaceplialus, and disappears so completely from the vertebric posterior to 

 this region, that it is impossible to say whether it is piimitively absent, or coossified. 

 The narrower anteroposterior diameter of the intercentra, and the absence of the 

 lateral costal articular groove would indicate its total absence. This element has 

 been observed by Fritsch in some Bohemian forms, and has been termed by him the 

 hypocentrum pleuralc. 



In the genera described by Dr. Fi'itsch from the Permian Gaskohle of Boluinia 

 some conditions have been described by him which differ considerably from those 

 mentioned above. The figures given by Dr. Fritsch are not entirely consistent 

 with each other, and ap])ear to have been taken from imperfect specimens. Thus in 

 SparcKjmites lacertinus Fi-.,'-' a neural arch is represented as resting on an intereen- 

 trum, while the arch next behind it rests on a hypocentrum pleuralc, or a divided 

 jjleurocentrum. It is not possible to be certain whether the neural arch stands on the 

 centrum or intercenti'um in the caudal region of Diplovertebron, as in some figures it 

 covers both (PI. 52, Fig. 2), and in others only one of these elements (PI. 50, Fig. 

 14). In the caudal I'cgion of the Diplovertebron ]mnct(Uum,i the vertebne have dif- 

 ferent forms, one being produced inferiorly apparently fi>r a chevron bone, and anotlu'r 

 posterior to it, being without prolongation, and without chevron. It is not possible 

 to be sure from Dr. Fi'itsch's descri[)tions and lignres whether the chevron bones ai'e 

 ai'ticulated, or arc continua, in these genera. 



Di-. Fritsch, however, determines the presence for the first time of the hypocen- 

 trum pleuralc, and describes it in the genera Chelydosaurus and Spheno.saui-ns.j: 

 With Gaudry, however, he regards tlu: intereentrum as the true centrum, and then 

 homologizes the hypocentrum pleuralc with the intereentrum of the Pelycosauria. 



I have attempted to sustain my interpretation of these homologies^ by reference 



• Fauna dcr Gaskolile tier I'ermiroruiations Bu;limeas, Hil. ii, II. 1, PI. .50, Kig. 16. 

 f Loc. cit.. PI. 50, Fig. 11. Dr. Fritsch believes llie inferior long bone to be a rib. 

 i Loc. cit., pp. 2.5-38. 

 § American Naturalist, 1885, p. 70; Proceeds. Amer. Philos. Soc, 1878, p. 522. 



