DESCRIPTIONS OF SPECIES. 377 



few comparativelj^ large scattering spines. This form has a wide distribution ia 

 the Mississippi Valley, and I have remarked it in Indiana, Illinois, Iowa, Missouri, 

 and Kansas. It undergoes many variations in minor particulars, but seems every- 

 where to retain the same general expression. Many specimens occur in which the 

 shape is not alate and the hinge line is no longer than the shell below. It is usuallj'' 

 difficult to determine whether this is due to breakage or is the natural shape, but in 

 some cases it can hardlj' be accidental. Oftentimes the curvature, instead of being 

 regularly arcuate, is angulated, the flattened visceral portions giving onto the lateral 

 parts by a sort of geniculation. The two forms occur together at the New Harmonjr 

 localit}' mentioned above, and are associated at most of the points from which collec- 

 tions have been obtained. The strise. which are never very strong, sometimes 

 become so faint that the shell looks almost smooth. In many cases they are apparent 

 over the posterior portion of the shell and become obsolete toward the margin. 

 More rarely they are obsolete over the visceral portions of the shell. The sinus is 

 often strong, but its variation is considerable. The peculiar submarginal ridges of 

 Marginifera are distinct in some specimens, but apparently lacking in others. I 

 have not been able to find them in mj^ material from the t3^pe localitj-. Although 

 the shells examined are thus variable, the variation is so inconstant that I would not 

 feel satisfied to distinguish any of them as separate species. In most cases, how- 

 ever, it is possible without much difficulty to discriminate between individuals with 

 regular curvature and those which are geniculate. I will provisionallj' make a 

 division of these shells upon this basis, distinguishing the geniculate ones as a variety 

 without, however, giving it a name. The other group is of course the typical one. 



In Colorado only the geniculate varietj' has as yet been found, and it seems to 

 be, as a rule, leather rare. My specimens, which ai'e for the most part in not very 

 good condition, agree closely with those from the Mississippi Valley. They add 

 nothing to the facts shown by the latter, and deviate onl}' so far as that they are 

 generally a little undersized. 



Another feature of these shells which va&y be worthy of mention is that I have 

 observed mau}'^ ventral valves to be covered externally, sometimes nearly- all over, 

 ))ut especially on the posterior portion, by a rough superficial deposit, which gives 

 them a peculiar scalj' appearance. This has been noticed at several localities in the 

 Mississippi Valley. I believe that it is not of mineral origin, but it may be parasitic. 



Of Procluctus spiendens also I have specimens from a number of localities in 

 Kansas. They are of a nearly uniform type, one whose characters correspond faith- 

 fully with those ascribed to P. splendens \iy Norwood and Pratten. The shell is 

 broad, strongly arched, often with the visceral area nearly flat and merging with a 

 geniculation into the lateral portions, this character being well shown by Norwood 

 and Pratten's figures." The sinus is deep, the ears well extended, concentric wrinkles 



" Loc. eit., pi. 1. figs. 5c, 5d. 



