LOWER SILURIAN LAMELLIBRANCHS. 635 



valve. Byssal opening rather small and narrow, lying in a sharply 

 defined, small, impressed area; in casts of the interior this impressed area 

 is considerably larger than in the shell itself. Ligamental area narrow, 

 striated; cardinal teeth very small; posterior lateral teeth not observed, 

 apparently wanting. 



A large specimen afforded the following measurements: Height, 

 from beaks to center of base, 73 mm.; height from posterior extremity of 

 hinge to same point, 60 mm.; greatest transverse diameter, 42 mm.; 

 thickness, 25 or 26 mm. 



The beaks are more attenuate and the hinge line shorter than in B. 

 radiata, Hall, sp. The general appearance of the shell is more like that 

 of the proposed Eridonychia apicalis, but the presence of cardinal teeth, 

 though unusually small for the genus Byssonychia, is quite sufficient to 

 distinguish it from that species and genus. For comparisons with B. im- 

 bricata see next description. 



Formation and locality: Middle beds of the Cincinnati group at a 

 number of localities in the vicinity of Cincinnati, Ohio. 



Byssonychia imbricata , n. sp. 



Plate 46, Figs. 4 and 5. 



Shell rather small, not known to exceed 35 mm. in height, moder- 

 ately convex, acuminate-ovate in outline, widest a little beneath the mid- 

 dle, .he greatest width about two-thirds of the length; beaks prominent, 

 not strongly incurved, approximate; hinge very short, upper half of an- 

 terior margin nearly straight; byssal opening small and very narrow. 

 Surface marked with from twenty-six to twenty-eight strongly rounded, 

 almost angular, radiating costae. These are crossed by distant, strong, 

 imbricating lines of growth, showing quite distinctly through the shell so 

 as to be reproduced on casts of the interior. 



The form of this species is almost exactly as in B. acutirostris, and, 

 were it not for the distant imbrications, it might be mistaken for the 

 young of that species. That such a view would be incorrect is shown 

 not only by the imbricating lines but by the smaller number of radiating 

 costae as well. These number, so far as observed, not less than thirty-six 

 in B. acutirostris and not over twenty-eight in B. imbri r ata. Compared 

 with B. radiata, it will be found that the hinge is shorter, the costse 

 fewer and the concentric lines very much stronger. 



Formation and locality: Middle beds of the Cincinnati group, near 

 the tops of the hills at Cincinnati, Ohio. 



Byssonychia (?) byrnesi, n. sp. 



Plate 47, Figs. 4 and 5 



Shell of medium size, obliquely ovate, with a short hinge, the cardi- 

 nal border often passing almost uniformly into the broadly rounded pos- 

 terior margin; valves strongly convex in the anterior and umbonal 



