LA.MELLIBRANCHIATA. 501 



Modlolopsidse.l 



pedal muscle scar. Posterior adductors large, very faintly impressed, situated less 

 than their diameter from the .posterior extremity of the hinge. Pallial line simple. , 

 Inner side of valves usually with one or two obtuse ridge-like thickenings extending 

 from the beaks obliquely backward toward the center of the ventral margin. 



Of the various genera included in this family in the scheme of classification on 

 page 485, 1 am satisfied that some of those preceded by a question mark will be sooner 

 or later placed elsewhere. No more satisfactory arrangement having suggested itself, 

 they were referred here, because their known characters agree with one or another 

 of the more typical genera. Thus, Aristerella, aside from its unequal valveci, 

 compares favorably with Eurymya, Gypricardella seems to be related to Modio- 

 morpha, and Endodesma to Modiolopsis and Cymatonota, while Psiloconcha, in a general 

 way, resembles Actinomya. But of Pyanomya tod little is known to venture an 

 opinion as to its ultimate placement, the only excuse for recognizing the genus in 

 this connection being that it would be even more out of place in any of the other 

 families. The position of Prolobella also is quite uncertain. 



Some of the species of Modiolopsis remind us so strongly of Modiola and Myoconcha 

 that we can scarcely escape the conviction that the latter genera, which are placed 

 respectively in the families Mytilidce and Prasinidce by Stoliczka and Zittel, have 

 really descended from Modiolopsis. Still, I am of the opinion that the paleozoic 

 types constitute a more natural grouping by themselves than can be attained by 

 any of the courses adopted heretofore. The position usually assigned to Modiolopsis 

 is near Modiola in the family Mytilidce, but Stoliczka and Zittel see greater resem- 

 blances with Myoconcha and therefore regard the genus as an early type of the 

 Prasinidce. But both of these families, the first in partiular, seem to me to include 

 heterogeneous material, and if they were revised according to the genesis of the, 

 Lamellibranchiata, I have no doubt their limits would be greatly modified. 



The first reason to influence me for the separation of Modiolopsis frOm the 

 Mytilidce occurred during a comparison with Myalina, Koninck, a genus that, while 

 it seems to be very justly associated with Mytiius, has no relation to Modiolopsis. 

 Indeed, according to my view, the progenitors of Myalina are to be sought for 

 among the Ambonyckiidce. 



Next, a comparison with recent species of Modiola proved that while a general 

 resemblance obtained there were still certain features in which the genera here 

 classed as the Modiolopsidoe agreed thoroughly among themselves and differed from 

 Modiola. Thus, in the latter, and the same is true of all the Mytilidce, the anterior 

 adductor impression is always smaller and the posterior one situated farther from 

 the cardinal margin as well as of a shape, including the prolongation formed by the 

 pedal muscles, never seen in the paleozoic shells under consideration. On the whole 



