66 PERMIAN FOSSILS. 



No. 1. DiTHYROCARis Permiana, noh. Tab. XVIII, fig. 1 a, b, c, d. 



INCH. 



1 

 26 



Length of carapace-valve. 

 Breadth — -^ 



Carapace-valve (left) oblong, convex, suddenly depressed along its anterior, 

 posterior, and outer margins ; bordered on its outer (left) edge by a slight ridge ; 

 smooth, faintly reticulated. Dextral {dorsal) edge nearly straight ; sinistral straight 

 in the middle, and curving off at each end to meet the dextral border. Extremities 

 obliquely rounded ; the anterior, in some specimens, almost squared ; posterior extremity 

 of the dorsal edge more acute than the anterior. 



Internally the valves exhibit considerable thickness at the posterior extremity. 



The valves of this species vary somewhat in the relative measurements of length 

 and breadth, and sometimes the valves have an increased breadth posteriorly. 



We have not met with a complete carapace ; and entire valves are of rare 

 occurrence ; casts, however, more or less perfect, are very common at Byers' Quarry. 



No. 2. DiTHYROCARis GLYPTA, noh. Tab. XVIII, fig. 12. 



INCH. 



Length of carapace-valve, J^ 

 Breadth — ^l 



Carapace-valve (left) oblong, convex, finely reticulated, bearing five curved 

 longitudinal ridges. One ridge on the outer (left) border, and a second, a little within 

 the first, extend from the junction of the dextral and anterior borders, following the 

 curvature of the edge of the valve, to the posterior angle of the dextral border ; the 

 third and fourth ridges follow a like course, at equal distances, within the former 

 ridges ; the third ridge faintly marked posteriorly, and the fourth rather sinuous ; a 

 fifth ridge is placed a little to the right of the fourth, but is interrupted anteriorly. 

 Sinistral border of the valve slightly incurved ; posterior extremity obliquely curved, 

 narrower than the anterior. 



In this and the preceding species the angle of the anterior extremity of the 

 dextral or dorsal border is more or less prominent, and not, as in other species, 

 removed so as to form a large central notch when both valves are in juxtaposition. 



From the same locality as the last. 



