400 Report on the Mollttsca of Long Ishnul, iY. Y.. 



left than to the right, the right side, however, heing quite convex in 

 its outline, while the left side is comparatively straight : roundedly 

 truncate b< fore, and very obliquely truncate behind, the left angle pro- 

 jecting. The stria 1 of growth arc prominent. The " smaller shell " is 

 anteriorly continuous with the dorsum of the " larger shell," generally 

 in such a manner that its upper surface is entirely exterior to that of the 

 " larger shell;" but in one specimen the anterior fourth is covered by 

 a thin plate of new shell, giving it the appearance of penetrating 

 the "larger shell" to that extent. Posteriorly it projects over the 

 margin of the "larger shell " to a variable but very small distance. 

 i i - dimensions, form, and markings are described below, but when con- 

 stituting a part of the full-grown shell its color is generally darker, 

 ami the markings less distinct than in the young shell. The interior 

 of the full-grown shell is whitish, and somewhat pearly. The smaller 

 shell is not simply soldered to it by the edges, but a plate of new shell 

 is carried OV< r the septum, the opening into the "smaller shell," 1 how- 

 ever, nol being diminished by it. The relative proportions of the sep- 

 tum and of this opening vary very much both in the young and the 

 adult shells, some young shells, although of unusually large size, hav- 

 ing onlj from one-eightfi to one-fourth of (he base covered by the sep- 

 tum, while one specimen, in which the " larger shell " was about two- 

 thirds developed, had a septum covering less than a quarter of the 

 aperture. All these forms of the young and old shells were found at 

 the same time (June, 1869) and in the same localities. 



The full-erown shell is ."> I millimetres lone: •">', tn. wide, and 1.1 m. 

 high. 



The voune shell is amber-colored, trauslucenl : sides nearly paral- 

 '•'"• ' ; lei, sometimes slightlv contracted in the middle : 



anterior end rather wider and more obtuse than 



ed 



d. 



,, . . „.. Apex very obtuse ai the posterior third of the 



length, and much inclined to the right. Concen- 



trie lin68 ("I" growth?) arc perceptible on the upper surface, as well 



number of radiating lines on the anterior end. Septum of lighter 



color, translucent, displaying distinct lines of growth, covering gene- 

 rally two third or ■ of the base ; the anterior edge straight, ami 



Qa. anterior end rat her wider and more obtuse thai 

 u| the posterior, often somewhat obliquely truncates 

 f \ on the left side; posterior end regularly rounded 



