820 THE PALEONTOLOGY OF MINNESOTA. 
(Patellide. 
When, however, we consider the great diversity of structure of the soft parts 
that can exist in shells looking so much alike, it surely seems highly improbable 
that any of the Paleozoic types could have continued on till the present time 
without being materially modified structurally. And yet it is most difficult to 
sustain this supposition by a comparison with the fossils. While the latter, so far 
as known, can in no case be said to be strictly identical with any of the recent 
genera, the resemblances in several instances are still very striking. Thus, while 
Tryblidium differs from Nacella and the other Patellide chiefly in having the mus- 
cular scars separate, the otherwise very similar proposed genus Archinacella 
approaches the recent forms even more closely in having the scars indistinguishably 
merged into a continuous narrow impression. Little is known of the muscular 
sears of Scenella and Helcionopsis, but comparing external characters they agree 
very well, the first with Acmwa or Lepeta, and the second with Helcion. 
While we admit freely that it may not be possible to prove that the Paleozoic 
Patellide are in all cases generically distinct from living types of the family, we are 
nevertheless fully convinced that such is the case. This conviction, as will be 
shown in the next paragraph, has something to support it besides the mere improba- 
bility of their identity. Obviously then we consider ourselves justified in proposing 
two new names and in retaining those which have already been proposed for those 
groups which it is convenient to distinguish. We were really forced to these views 
by the miserable failure of our efforts to distribute the Paleozoic species among the 
recent genera, After repeated endeavors, the results being different every time, we 
gave it up as being, to say the least, impracticable. 
The distinctive evidence. referred to in the preceding paragraph is shown in 
three specimens before us. It consists namely ofa pair of rostral muscular imprints 
which seem not to belong to the usual ring of scars and which we do not find 
Fig. 1.—a, dorsal view of a cast of the interior of Archinacella powersit U. & 8., showing the rostral 
scars at A; b, dorsal view of a partial cast of the interior of Tryblidiwm unguis Lindstrom; c, apical portion 
of same in a direct view to show the rostral scars; d, dorsal view of a cast of Lepetopsis, 5p. undet., from 
Kansas City, Mo, showing muscular imprints very satisfactorily. 
