702 E. C. CASE 



Later several complete crania of the genus were obtained from Texas and 

 described in detail by Professor Cope, Trans. Am. Phil. Soc, 1884, pp. 572- 

 590, 1 plate (Pal. Bull., No. 38). In the American Naturalist of the same 

 year, p. 413, the genus was made the type form of the new order Ichthyotomi 

 of the Elasmobranchii. 



The form is now quite usually recognized as belonging to the genus Pleura- 

 canthus Ag., one of the common forms of the Carboniferous and Permian 

 faunas of Europe and America. 



[No. 6504.] 



Thoracodus emydinus Cope. 



Thoracodus emydinus Cope, 1883, Proc. Acad. Nat. Sc, Phil., 



p. 108. 

 Thoracodus emydinus Woodward 1889, Cat. Foss. Fishes 



Brit. Mus., Pt. I, p. 39. 



"The form of the tooth or jaw on which this genus is proposed, reminds 

 one of that of a Diodon, and also of one half of that of a Janassa. It appears 

 to be the half of a bilateral plate, which is divided on the middle line by 

 suture. Its form is somewhat that of the anterior part of an episternal bone 

 of a tortoise. It consists essentially of a smooth border, separated from the 

 remainder of the tooth by a transverse groove. The interior portion is, on 

 the superior face (if the piece belong to the inferior jaw, and vice versa), 

 transversely ridged and grooved, after the manner of the genus Janassa'' 



Specific characters : " The smooth border is wide above and below. Its 

 edge is produced into a median projection, which is decurved. On the infe- 

 rior surface it is marked by shallow grooves, which radiate from the groove 

 which bounds it posteriorly, extending nearly to the free edge. Posterior to 

 the bounding groove, the surface is smooth. The posterior surface above 

 has its grooves concentric with the curved free margin. The ridges are nar- 

 row, and step-like in position, presenting their free edges backwards. There 

 are no grooves other than these steps. They have an angular curve oppo- 

 site to the angle of the free margin, and at the angle the groove which sepa- 

 rates them is narrowed, while it widens at other points. Free edge of border 

 thickened ; surface everywhere smooth." 



MEASUREMENTS. 



"Length of fragment transversely - - - - .014"' 



Length of fragment antero-posteriorly - - .011 



Width of border area at median suture - - .005 



Seven cross ridges ------ .005 



Thickness at suture at cross ridges - - - .002 " 



[This specimen is missing from the collection.] 



