OF CONCHOLOGY. 81 



erably in different specimens. The hoamal valve appears to have 

 had them more prominently developed than the other, in all 

 cases. The median septum of this valve also seems to be more 

 extended anteriorly than that of the neural valve. The internal 

 tubular processes of the haemal valve are, as Lindstrom represents 

 them, abruptly raised from the surface of the valve or even over- 

 hanging it, so that they represent two cylinders side by side, 

 with a gutter between them, which represents the top of a sep- 

 tum which is prolonged beyond their ends, anteriorly. In the 

 neural valve, however, these cylinders are less prominent, and 

 in some of the specimens the structure loses' the appearance of 

 cylinders and assumes the aspect of a plate or lamina, supported 

 by a median septum and passing off into the body of the valve 

 at its lateral edges, as described in the generic diagnosis of Trini- 

 erella in my first paper. In the Ohio species the internal arrange- 

 ment differs somewhat in detail. Instead of the anterior ends of 

 the cylinders being abruptly truncate, as in Lindstrom's figures, 

 the upper edge is anteriorly produced and rounded off at the sides, 

 so that it projects beyond the lower termination of its respective 

 cylinder, while the septum is very prominent and also rounded 

 and projecting above, over its attachment to the shell below. 



The following provisional diagnosis comprehends the most 

 evident characters. 



TRIMERELLA, Billings, 1862. 



= Trimerella, Billings, Pal. Fos. Geol. Can. i, p. 166, June, 1862. 



> Trimerella, Dall, op. cit. p. 160, 1870. 



? Trimerella, Lindstrom, op. cit. p. 253, 1867. 



? Gotlandia, Dall, (after Lindstrom) op. cit. p. 160, 1870. 



Shell allied to Obolus. Neural (ventral) valve with a pro- 

 duced beak bearing a false area of which the middle part is more 

 or less concave, and the whole of which is crossed by strong im- 

 bricated lines of growth. The plane of the area is at an obtuse 

 angle with the plane of the margin of the valves. Haemal (dor- 

 sal) valve with a more or less incurved apex, on either side of 

 which is a small transversely striated area. Margins smooth, 

 flat, thickened, rarely flexuous. Neural valve with a subtriangu- 

 lar plate, proceeding forward from beneath the hinge margin, 

 more or less concave in the median line and more or less elevated 

 above the body of the valve at its lateral edges. Supported be- 

 low by a median septum which is usually prolonged beyond the 

 anterior edge of the plate. 



Hgemal valve with a similar arrangement, except that the plate 

 is more convex on each side of the median line, more elevated 



