276 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.52. 



lost, together with the greater portion of the caudal, and the recum- 

 bent rays of the dorsal have become closely appressed and are in part 

 concealed by matrix. No other examples of this species besides the 

 two that are here mentioned have come to light. 



Formation and locality. — Coal Measures; Mazon Creek, Illinois. 



Family PLATYSOMATIDAE. 



Three species of the genus Platijsomus have been established upon 

 the evidence of a single specimen each from the Mazon Creek locality, 

 all unsatisfactorily illustrated and described; and a fourth species 

 (P. palmaris Cope) has been made known from the Permian of the 

 southern part of Indian Territory. The species founded upon Mazon 

 Creek types have been named as foUows : P. orbicularis, P. circularis, 

 and P. lacovianus. The first mentioned of these has been reinves- 

 tigated by the present writer and transferred to the genus Cheirodus. 

 The holotype of P. circularis is preserved in the Museum of Illinois 

 State University at Urbana, and the counter impression of the same 

 specimen in the American Museum of Natural History, labeled m 

 Newberry's handwriting. 



Genus CHEIRODUS M'Coy. 



In this genus the trunk is rhombic, pectoral fuis insignificant, and 

 pelvics absent; the scales are deep and slightly imbricated; margin 

 of mouth tootliless, pterygoid and splenial with two denticulated long- 

 itudinal ridges. 



CHEIRODUS ORBICULARIS (Newberry and Worthen). 



Plate 9, fig. 2. 



Platysomus orbicularis Newberry and Worthen, Palaoont. Illinoi.=, vol. 4, 1870, 



pi. 3, fig. 1 (no description). 

 Cheirodus orbicularis Eastman, Bull. Mus. Comp. Zool., vol. 39, 1903, p. 193, pi. 5, 



fig. 52. 



It is not known where the holotype of this species is now preserved. 

 A poor figure of this unique specimen was given by the original au- 

 thors, unaccompanied by any textual description. Upon the evi- 

 dence of a number of specimens belonging to the Peabody Museum of 

 Yale University, one of which was labeled in Newberry's handwriting 

 as pertammg to this species, an amended defuiition of it was drawn 

 up by the present writer, wliich may be here restated: 



A small species, attaining a maximum length of about 4.5 cm. Trunk deep, orbicu- 

 lar in outline, the dorsal margin elevated into a prominent peak at about its middle 

 point, and the ventral margin angulated to a somewhat less extent at a point aboiit 

 midway between the branchial apparatus and the narrow caudal pedicle. Facial 

 contour of head steep, cranial plates granulated and striated; the head with opercular 

 apparatus contained about two and one-half times in the total length to the base of the 

 caudal fin. Dorsal and anal fins arising at a considerable distance behind the mar- 



