LOWER SILURIAN LAMELUBRANCHS. 633 



57 mm., 87 mm. and 30 mm. Beaks rather prominent, triangular, cari- 

 nate, curving very slightly forward, and rather widely separated in casts. 

 Anterior outline nearly straight, the margin projecting a little in the 

 lower part; base strongly convex, posterior margin broadly rounded; 

 hinge line abouftwo-thirds as long as the middle length of the shell, rang- 

 ing at an angle of about 95° with the anterior margin. Byssal opening 

 large, in casts appearing as an acutely elliptical low prominence, situated 

 about its length beneath the summits of the beaks. Costse of moder- 

 ate strength ; their number, though not certainly determined, is not less 

 than fifty. Posterior adductor scar and pallial line as shown on plate 45. 



The shell of this species has not yet been observed, but the casts are 

 not uncommon, and with their broadly flattened and nearly straight an- 

 terior sides are so easily distinguished from all the other species of the 

 genus, except B. robusta, Miller, sp., that a name for them has long been 

 desirable. In 1880 (loc. cit.) Mr. Miller referred these casts to his species 

 robusta, but in a recent conversation he admitted that they probably be- 

 longed to a distinct species. B. robusta, as figured, is relatively not so 

 high and has coarser rays, their number being only about forty, while in 

 B. richmondensis there are at least ten more. B. cultrata is closely related, 

 but differs decidedly in its outline, being a wider shell and not so convex. 

 The flattening of the anterior side also is confined to the upper part, 

 while in the lower part the outline curves forward in a much greater 

 degree. 



Formation and locality : Associated with Rhynconella dentata, Hall, 

 and Ortonella hainesi, Miller, sp., in the upper beds of the Cincinnati 

 group at Richmond, Indiana. 



Byssonychia prcecursa, n. sp. or var. 



Plate 45, Figs. 1 and 2. 



This form I regard as a small forerunner of B. richmondensis, B. ro- 

 busta, and possibly of B. cultrata as well. The shape agrees best with B. 

 richmondensis, the principal difference being in the hinge line which is 

 always longer and sometimes quite equal to the greatest length of the shell. 

 The number of the costse varies from thirty-eight to forty-two, the aver- 

 age number being the same as for B. robusta and ten less than in B- 



richmondensis. 



In the number of costae and in the outline B. prescursa is very much 

 like the typical form of B. radiata, Hall, sp. * As a rule, however, the 

 latter is a little more oblique, the hinge shorter and the central part of its 

 valves a trifle wider. But the principal difference lies in the flattening 

 of the anterior side in B. prescursa. 



* In twenty specimens of the Cincinnati form of B. radtata the number of the 

 rays varied between thirty-six and forty, while a good example from an unknown 

 '.ocality in New York has thirty-six. 



