LAMELLIBRANCHIATA. 501 
Modilolopside.] 
pedal muscle scar. Posterior adductors large, very faintly impressed, situated Jess 
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 455, [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 valves, 
compares favorably with Hurymya, Cypricardella seems to be related to Modio- 
morpha, and Endodesma to Modiolopsis and Cymatonota, while Psiloconcha, in a general 
way, resembles Aetinomya. ‘But of Pyanomya too 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 Mytilide and Prasinidw by Stoliczka and Zittel, have 
really descended from Modiolopsis. Still, | 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 Mytilidw, but Stoliczka and Zittel see greater resem- 
blances with Myoconcha and therefore regard the genus as an early type of the 
Prasinide. 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 
Mytilide occurred during a comparison with Myalina, Koninck, a genus that, while 
it seems to be very justly associated with Mytilus, has no relation to Modiolopsis. 
Indeed, according to my view, the progenitors of Myalina are to be sought for 
among the Ambonychiide. 
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 Modiolopside agreed thoroughly among themselves and differed from 
Modiola. Thus, in the latter, and the same is true of all the Mytilide, 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 
