192 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



PLATYSOMATIDAE. 



Three unique specimens from the Mazon Creek locality, all more or less 

 imperfectly figured and described, have been referred to as many species of 

 Platysomus, and a fourth species has been described by Cope (P. palninris) 

 from the Permian of the southern part of Indian Territory. The characters of 

 the so-called P. orbicularis Newberry and Worthen have never been defined, 

 and the type specimen is here regarded as pertaining to Cheirodus. 



Platysomus circularis Nkwberry and Worthen. 



(Plate 5, Fig. 51.) 



1870. Platijsomus circularis Newberry and Worthen, Pal. Illinois, Vol. IV., p. 347, 

 pi. IV., Fig. 2. 



Type. — Complete fish ; Illinois State University, Urbana. 



A very small species, attaining a maximum length of about 4 cm. Outline 

 of body elliptical, greatest depth of trunk exceeding its length from the pec- 

 toral arch to the base of the caudal fin, and more than twice as long as the 

 head with opercular apparatus ; dorsal margin gibbously rounded from the 

 occiput to the narrow caudal pedicle, ventral margin regularly rounded. Dor- 

 sal and anal fins arising considerably behind the middle of the back, relatively 

 high, and extending close to the origin of the caudal fin. Scales finely 

 striated, the striae being parallel, even, and regular, vertical ou those situated 

 nearest to the ventral margin in advance of the anal fin, but oblique on the 

 remaining longitudinal rows. 



In the original figure of this species, the squamation is very distinctly 

 shown, and the scales are described as being '' oblong in outline, smooth, those 

 on the sides three to six times as high as long." An examination of the type, 

 however, kindly permitted by Prof. C. W. Rolfe, reveals the fact that the 

 scales are very inaccurately drawn, and that their striated condition Avas over- 

 looked by the authors. Several examples, clearly belonging to this species, 

 are preserved in the Yale Museum, one of which has been selected for illus- 

 tration in the accompanying plates, and the definition of the species has been 

 amended in conformity with characters displayed by the additional material. 

 The dorsal and anal fins are stated by Newberry and Worthen as consisting of 

 forty and thirty dermal rays, respectively, but it is probable that even more 

 than this number were present. 



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



