SYSTEMATIC REVISION 75 



noticed, the ribs of Cricotus are single-headed. The centrum and intercen- 

 trum take neariy equal part in the support of the large head of the sacral rib. 

 For a comparison with the other species see the original description of 

 C. crassidiscus. 



Cricotus gibsonii Cope. 



Cope, Proc. Am. Phil. Soc, vol. xvii, 1877, p. 185; Am. Nat., vol. xviii, 1884, p. 39. 

 Case, Journ. GeoL, vol. viii, 1900, p. 709. 



Type: Several vertebrae of similar form, Nos. 6521 and 6522 University 

 of Chicago. From near Danville, Illinois. 



Original description: "The vertebra which is best preserved and which 

 may be regarded as typical is probably from the caudal series, and is thus 

 well contrasted with the corresponding typical vertebrae of the longer known 

 species. 



"On this vertebra there is no trace of a diapophysis, and the neurapophy- 

 sis rises from the external side of the superior face. The wall of the neural 

 canal is not preserved, but the inference is that the diameter of the latter is 

 large. This fact and the absence of definite chevron articulations leads me 

 to doubt the caudal position of the vertebra; but the usual marks of the 

 dorsal and cervical vertebrae are totally wanting from it. As in C. hetero- 

 clitus, the foramen chorda dorsalis is large, its diameter being one-third of 

 the total. The articular faces descend steeply into it, that of one extremity 

 more so than the other. The rim of the latter face is beveled outwards, the 

 plane thus produced appearing on the inferior face something like the united 

 faces of the chevron bones. 



"The centrum is a little deeper than wide, and the inferior face is trun- 

 cate so as to give a subquadrate outline. The inferior plane is concave, the 

 concavity being divided by a longitudinal rib. The sides are somewhat con- 

 cave, with a longitudinal rib at the middle. Diameters of centrum : vertical 

 .010 m.; transverse .009; longitudinal .008. Width of the inferior plane .005 ; 

 width above, including neurapophyses .008. 



"As compared with C. heteroclitus, this species differs in the presence 

 of parallel ridges inclosing a median fossa on the inferior side of the centrum. 

 The small size may be here considered, but it is uncertain whether the two 

 animals represented by the vertebrae are full grown." 



Revised description: It is doubtful whether this species could be dis- 

 tinguished from the others or not. It is very possible that the vertebrae are 

 from some portion of the column not represented in other specimens. Cope 

 does not mention it in the analysis of species given in 1884 (38), and appa- 

 rently regarded it as included in the description of C. heteroclitus. 



Cricotus heteroclitus Cope. 



Cope, C. heteroclitus, Proc. Phil. Acad. Nat. Sc, 1875, p. 405. 



C. discophorous, Proc. Am. Phil. Soc, vol. xvii, 1877, p. 186. 



C. heteroclitus, Proc. Am. Phil. Soc, vol. xvn, 1878, p. 523. 

 Case, C. heteroclitus, Journ. Geol., vol. vin, 1900, p. 708. 



Type: The same as the genus. No. 6518 University of Chicago, Type 

 of C. discophorous. 



Original description: "The surface of the sides of the centrum (inter- 

 centrum) is marked with a few coarse shallow longitudinal grooves, which 

 run into shallow reticulations of weak raised lines. The neurapophysis is 

 sharp-edged in front, and with some ridges externally at the base. 



