NATURAL SCIENCES OF PHILADELPHIA. 369 



terior end, being broadly rounded in advance and gradually tapering behind. 

 Surface marked by irregular lines of growth, placed very closely, sometimes 

 most distinct on the basal half of the shell. 



Length from the anterior to posterior end, -3 in.; greatest width, at right 

 angles to first measurement, -18 in.; depth of one valve, -09 in. These measure- 

 ments are from a small single valve. I have before me another specimen im- 

 bedded in a piece of shell of one of the large Mercenaries of the Miocene, pro- 

 bably of Jamss River, Va., which is more than twice as large. From marks on this 

 fragment, it appears to be gregarious. Compared with R. dubia (recent) as 

 figured by H. and A. Adams, this shell is broader posteriorly, more curved on 

 the cardinal side, and the open spice between the two valves, in front, com- 

 mences more abruptly and is continued further posteriorly. The beaks are 

 more nearly terminal, the anterior portion of the shell being less produced. 



Sphenia Turton. 



S. bilirata. Shell elongated subquadrate, convex. Beaks nearly termi- 

 nal, small, incurved. Umbones small, depressed or undulated by a faint median 

 depression which usually extends about half the length of the shell, becoming 

 obsolete. Anterior extremity most prominent near the basal nurgin. Poste- 

 rior end abruptly truacated. Cardinal and basal margins nearly parallel, nar- 

 rowest posteriorly. The basal margin is straight or very slightly emarginate. 

 From the beak extend two distinct ridges, both very much imbricated in the 

 young shell, less so in the more advanced state. The most anterior of these 

 runs directly to the posterior basal angles, the other is intermediate between it 

 and the cardinal edge, both ending rather abruptly when the shell is about half 

 of its full size. Surface closely and irregularly marked by heavy folds. 



Length, -2 in. ; width, -35 in. ; greatest diameter (variable) "15 in. 



Santa Barbara, Cal. 



Tbllina Linn. 



T. euryterm a. Shell wide, nearly equilateral. Beaks small, acute. Ex- 

 tremities broadly and nearly evenly rounded. Hinge line slightly excavated in 

 advance of the beaks, straight and sloping behind. Surface smooth or marked 

 by faint lines of growth. Hinge small. 



Length, -5 in. ; width, -95 in. ; height of single valve, -1 in. 



Allied to T. sericea Con., but can be distinguished by the anterior end, which in 

 that species is narrow and even subacute, being in this as broad as the posterior 

 end. The beaks are more nearly central than in that species. Having nearly 

 the same general form as Psammobia lintea Con., the beaks are more elevated, 

 the ends more even and the surface smooth, while it is marked by elevated lines 

 in the latter species. 



From the Eocene of Vicksburg, Miss. 



Venus Linn. 



V. rhy som i a. Shell small, rounded subtriangular. Beaks small, incur- 

 ved, and placed about a third of the width, from the anterior end. Posterior 

 cardinal margin nearly straight; posterior extremity narrow, rounded. Surface 

 highly polished, marked by obsolete lines of growth, and radiating lines so in- 

 distinct as only to be visible through a lens by the aid of reflected light. Hinge 

 short, teeth compressed. Pallial sinus deep and broadly rounded at its base. 

 Internal margin smooth, 



Leugth, -2 in. ; width, -25 in. ; depth of valve, '06 in. 



From Santa Barbara, Cal. Miocene (?) 



I have seen three valves of this species, the measurements above being from 

 the largest. I never saw fossils presenting more the appearance of recent shells 

 than these specimens. Except for a slight change in color, they could not be 



1861.] 



