Tropidoi.ki'tus Fauna at Canandaicua Lake, N. V. 97 



vex, had no striixi, and were strongly ])unctate. The interior of the 

 brachial valve was characterized as bearing three divergent ridges in 

 front of the muscular area. 



Stropheodonta nacrea Hall, from the Corniferous and Hamilton, an 

 unnamed species from the Corniferous, and StropJiomena lepis Bronn. 

 of the Middle Devonian from Eifel, Belgium, and the Asturias, were 

 named as belonging to this division. 



It has been shown that the development of the hinge structure, form 

 of shell, and convexity of valves is very similar in the three species 

 just described. The points of greatest difference were : first, in the 

 striae — .S". iuirqiiistriata produced new striae by implantation; S. per- 

 plana by both implantation and bifurcation ; and P. iowa'cnsis has 

 normally no strice at any stage of development ; second, in the scars 

 of the muscles — S. inccquistriata has a type very different from that 

 of the other two, and, in the ventral valve, there is a change in the 

 form of the muscles during the ephebic and gerontic stages. In the 

 other two species no such change has been observed to occur. 



Comparing the scars in the ventral valves of S. jiinia, perplana, de- 

 missa and P. iowa'cnsis, it is at once seen that they are very similar. 

 In each, the diductors are broad, flabelliform, separated by a low 

 septum and bounded on their posterior-lateral edges by more or less 

 papillose ridges. In S. demissa and P. iozvacnsis, they are more 

 sharply marked in front than they are in the other two species. Be- 

 tween the diductors are the elongate scars of the adductors, two pairs 

 in each case. In the dorsal valves of S. demissa, S. poplana and 

 P. iowa'eiisis, there is more variation in the form of the scars, but it is 

 more a variation in the limiting structures than in the shape of the 

 scars themselves. In each, there are two pairs of scars, one pair 

 somewhat anterior to, and between the other pair. In S. demissa and 

 P. ioiva'cusis they are bounded by a ridge in front. In S. perplatia, 

 the anterior margin is more indefinite. The interior of the dorsal valve 

 of P. iowa'cnsis should be compared with that of Stropheodonta pro- 

 funda Hall from the Niagara (Pal. N. Y., Vol. VIII. I, pi. 20, fig. 

 30). In that species the form of the scars is almost exactly the same 

 as in the Hamilton species, and in front of the scars there are two 

 short, curved ridges and a ])rolongation of the median septum. Hall 

 thus describes this species. ' ' Shell large, full grown individuals having 

 a width of 60 mm. and length of 40 mm. deeply concavo-conve.x. 

 ^'entral valve very convex, hinge line narrow, foramen triangular, 



