68 PALEONTOLOGY. 



ventral valve, about 4.90 inches; breadth of the gibbous part of umbonal 

 region, exclusive of the ears, about 2.50 inches. 



Of this Frodudus, there are six or eight specimens in the collection 

 from various localities, but unfortunately they are all more or less mutilated 

 or distorted. After numerous careful comparisons, I liave been unable to 

 arrive at any very satisfactory conclusions in regard to its specific relations. 

 At a first glance, it reminds one of some of the larger coarsely- costated va- 

 I'ieties of P. semireticulatus. It evidently differs, however, inseveral respects 

 from that species; being not only more coarsely costated, and more deeply 

 sulcated, but it likewise differs in having numerous little spines scattered 

 over the whole of the ventral valve; while the mesial internal ridge of its 

 dorsal valve (see fig. 6 &) differs in being bifid as in P. scabriculus. In most 

 of its external characters at least, excepting its narrower form, it seems to 

 agree pretty well with the description of a form described by Professor Swal- 

 low, from the Upper Coal-Measures of Kansas, under the name P. Calhomi- 

 ianus var. Kansasensis ; and yet it appears to be specifically distinct from 

 the Kansas specimens that I have always identified with the form described 

 by Professor Swallow, as well as from all of the others I have ever seen 

 from the same localities as those named by him. It appears also to be re- 

 lated to P. Ivesii of Dr. Newberry, from Arizona, but is more produced 

 anteriorly ; and, so far as I have been able to see from examinations of the 

 interior of the dorsal valve of that form, its mesial ridge is not divided as 

 in this shell, which is also narrower in the umbonal region. 



I suspect that it will be found to be an unnamed species ; but, without 

 better specimens for comparison, I hardly feel justifiable in naming it as a 

 new species. If distinct from all the allied forms, it might be called P. 

 lof)gus. 



Locality and position. — Fossil Hill, White Pine Mountains; Eailroad 

 Canon, Diamond Mountains ; West of San Francisco Mountain, etc. 



