K I- I'TILIAN AGE. J UR A8SIC PE RIOD. 87 



li, 2.05 inches ; diameter from the dorsal margins, at right angle* to the length, near the middle of the 

 shell, 0.34 inch ; greatest convexity at the same point, 0.80 Inch. 



VoUtlla format*. 

 A. Dorsal view. B. Side view. 



This species belongs to the irregularly proposed genus Perna, Adanson, = Modio- 

 /.-. Lamarck, and will fall into Swainson's section Brachyoftontes. It seems to be 

 \cr\ do.xeK related to the European Volsclta cancellata (= Mytilus cancellatits, Gold- 

 I'n*-. 1'etret". (ierm. tome ii, pi. 131, fig. 2), and may possibly prove to be identical, 

 when we can have an opportunity to compare a good series of each. The five or six 

 specimens, however, of the form here described, that we have seen, all present the 

 following differences from Goldfuss' figure : In the first place they are narrower from 

 the dorsal to the ventral margins, more narrowly rounded at the posterior extremity, 

 and have slightly more prominent beaks ; while their antero-ventral region is a little 

 more convex, and their basal outline more arcuate. The surface markings of the 

 species under consideration are very similar to those of Goldfuss' species, though 

 his enlarged figure represents the concentric strite crossing the radiating lines as 

 being more distinct and regular than on our shell. Again, he neither figures, nor 

 mentions in his description, the fine obscure transverse striae seen on the ventral 

 half of our species, though these are so indistinct that they might be easily over- 

 looked ; indeed they seem to be rather dependent, in some way, upon the structure 

 of the shell, than properly surface markings. 



"We observe D'Orbigny cites Modiola camrllufn, of Roemcr, and his own M. 

 AV /<//>/.////<, from the Jura of Russia, as synonymous with Mi/tilns cancellatw, 

 Goldfuss. Without knowing to what extent Goldfuss' species may vary, we cannot 

 leel prepared to express a positive opinion in regard to its relations to the forms 

 named by Rocmer and D'Orbigny; though judging by their figures, we would be 

 inclined to doubt the identity of these shells. At any rate, Roemer's and D'Orbigny's 

 figures are very unlike the form now. before us, which is remarkably uniform in its 

 characters. 



Locality and jxwition. From the Jurassic beds of Big Horn Mountains, Dakota 

 Territory. (Type 1882.) 



FAMILY AKCID^E. 



Shell eqni valve- or -eubequi valve, not pearly within, closed or gaping 

 below, usually gibbous; surface frequently ornamented with radiating 

 costee or strite. Hinge straight or arched ; provided with a more or less 

 elongated posterior and anterior lateral tooth, which are divided trans- 

 M-rsely, obliquely, or longitudinally, into small interlocking, short, or 



