79 



Orthis Electra. (N. sp.) 



PARVA?— Billings, Can. Nat. Geol., vol. 6, pp. 312, 315, 346. 



a I c 



Fig. 72. 



Fig. 73. 



Fig. 74. 



Fig. 72. — Orthis Electra, — a, b, Ventral and side views of a specimen with the 

 valves in place ; c, view of a detached ventral valve ; d, view of 

 a dorsal valve. 



73. — Hippolyte, — a, b, c. Different views of a small specimen ; d, e,f, 



a large specimen restored from detached valves. 

 74. — O Evadne, — a, b, c, d, Different views of a specimen. 



Description. — About the size and nearly of the shape of 0. perveta 

 (Conrad), to which species it is closely allied. Shell semi-oval or suborbi- 

 cular ; greatest width about the mid-length ; cardinal angles a little more 

 than 90°, rarely rectangular, sometimes appearing to be slightly rounded ; 

 hinge-line usually a little less than the greatest width of the shell ; sides 

 gently convex, sometimes straight for a short distance below the cardinal 

 angles ; anterior angles rounded ; front margin usually convex, but some- 

 times with a portion in the middle straight. Ventral valves strongly and 

 narrowly convex along the middle in the upper half, descending with a 

 somewhat flat or gently convex slope to the sides and cardinal angles, the 

 latter not much compressed. The front half of the valve becomes more 

 broadly convex, but in general the carination, which commences at the 

 beak, can still be perceived, although nearly obsolete. Beak small, only a 

 little projecting over the hinge-lme, depressed to about one third the 

 height of the shell ; area small, at its base forming an angle of a little 

 over 100° with the plane of the lateral margin, strongly concave in the 

 upper part ; foramen small, about as wi^le as it is high. 



Dorsal valve very gently convex, with an indistinct wide shallow mesial 

 sinus, which in some specimens is scarcely perceptible. Area small, form- 

 ing an obtuse angle with the plane of the lateral margin ; foramen small 

 but well defined to the very point of the beak ; the latter scarcely distinct 

 from the cardinal edge. 



In the interior of the ventral valve the muscular impressions are not 

 distinguishable in any specimen that I have seen, but just in front of the 



