1834.] Oo7 [Cope. 



F. Cummins in the Permian beds of Texas. Parts of the jaws with two 

 of its teeth are preserved. The lower jaw is distinguished from that of 

 the D. compressus by its small transverse as compared with its other di- 

 ameters. The ramus is quite compressed, and is not thicker at the inferior 

 edge than the superior, and is slightly concave on the inner side. Its ex- 

 ternal face is nearly vertical. The angle is rounded forwards, and there 

 is no angle behind the cotylus, which is raised above the superior line of 

 the ramus. The cotylus is rather large, and has a shallow anterior supe- 

 rior, and a posterior subposterior facet. There is no indication of a coro- 

 noid process. The inferior edge of the ramus is swollen on the outer 

 side, below the anterior border of the condyle, so as to mark with the 

 thickened posterior edge of the ramus a fossa in the position of the mas- 

 seteric. 



The teeth are pecular in the form of the root (Figs. 8-9). This part has 

 no (interior projection, and the posterior portion is a flat, thin-edged plate, 

 wider than long. It carries a button, but no notch. There is a minute 

 median denticle. The form of the root is thus very different from that of 

 the tooth of the D. compressus (figs. 5, 7). 



Measurements. M. 



Depth of ramus at cotylus (vertical) 062 



" " " 120 mm. anterior to cotylus. .048 



Transverse diameter at the same point .009 



Long diameter (oblique) of cotylus 031 



anteroposterior 011 



Diameters of base of tooth Uran8verse 037 



( anteroposterior .0048 

 Diameters of crown of lateral denticle 1 tringvel . sl () np 



I call this species Didymodus platypternus. Should the name Didymodus 

 be found hereafter to apply to species of Pleuracanthus, the latter generic 

 name must be used for this species. 



III. Historical. 



In 1837 Prof. Agassiz (Poiss. loss., iii, 66), described a spine which 

 he believed to have belonged to a fish like the sting-rays, as Pleuracanthus 

 Imvissimus. The only example was obtained from the Dudley Coal field. 



In 1845 Prof. Agassiz (Poiss. foss., iii, 204), made known certain 

 teeth, which he referred to sharks of the family of Hybodonts. Two spe- 

 cies were distinguished, D. gibbossus and D. minutus. Both were obtained 

 from the English Coal measures. 



In 1848 Prof. Beyrich (Berichte vernandl. k. Preuss. Akad. wiss. , 

 1848), proposed the generic name Xenacanthus for a German Carbonifer- 

 ous form, referred to Orthacanthus by Coldfuss (1847), but which ap- 

 proached nearer to Pleuracanthus. 



In 1849 Dr. Jordan (Jahrbuch fiir Min. u. Geol., p. 843), described, 

 under the name Triodus sessilis, a form subsequently ascertained to be 

 identical with the Xenacanthus. 



