820 THE PALEONTOLOGY OF MINNESOTA. 



[Patellida;. 



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 Patellidcv 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 

 scars of Scenella and Helcionopsis, but comparing external characters they agree 

 very well, the first with Acmcea or Lepeta, and the second with Helcion. 



While we admit freely that it may not be possible to prove that the Paleozoic 

 Patellidce 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 of a pair of rostral muscular imprints 

 which seem not to belong to the usual ring of scars and which we do not find 



Fig. 1. o, dorsal view of a cast of the interior of ArcMnacella powersi U. & S., showing the rostral 

 scars at A; b, dorsal view of a partial cast ol the intt-rior of Tryllidium unguis Lindstroru; c, apical portion 

 of same in a direct view to show the rostral scars; d, dorsal view of a cast of I^jietopsis, sp. undet., from 

 Kansas City, Mo., showing muscular imprints very satisfactorily. 



