LOWER SILURIAN LAMELLIBRANCHS. 631 



Byssonychia alveolata, n. sp. 



Plate 48, Figs. 1-3. 



Shell of medium size, moderately convex, obliquely acuminate-ovate, 

 wider than usual, with the basal half of the outline semicircular; cardinal 

 margin somewhat shorter than the middle length of the shell; umbones 

 full, beaks but little incurved, separated; ligamental area very large; 

 beneath the beaks the anterior side is impressed, forming an obscurely 

 defined subcordate lunule, in the lower part of which the byssal opening 

 is situated. Surface marked by about fifty rounded radiating costse. 



The large ligamental area indicates relationship with B. grandis and 

 B. obesa, both of which are restricted to a higher horizon. The first is 

 sufficiently distinguished by its carinate umbones; the second is a more 

 erect shell, with a shorter hinge line, narrower area, and differently 

 shaped byssal depression. The w T ide area should separate the species at 

 once from B. radiata with which a careless collector might confound it- 



Formation and locality : Middle beds of the Cincinnati group, Cin- 

 cinnati, Ohio. 



Byssonychia grandis, n. sp. 



Plate 46, Figs. 6-9. 



Shell large, ventricose, subquadrate, the length and height as ten is 

 to thirteen; anterior margin sinuate above, broadly convex in the lower 

 two-thirds; outline of basal half semicircular; hinge line about two-thirds 

 as long as the shell is at the middle of the height. Beaks projecting less 

 than usual, carinate, flattened on the anterior side; apices separated widely, 

 the intervening space being occupied by a broad, striated ligamental area. 

 Upper part of the anterior side with a broad and deep impression in the 

 bottom of which lies the byssal opening. Surface marked with about 

 forty radiating costse. These are rounded and broad in the lower half of 

 the shell. Posterior lateral teeth small, two, situated near the extremity 

 of the hinge. 



This species probably attained a larger size than any other known. 

 It may be equaled in this respect by the associated B. cultrata, a species 

 that resembles it in its outline and in having carinate umbones as well. 

 But the present species is readily distinguished from that one by its 

 greater convexity, coarser and therefore fewer costae, and by the large 

 depression around the byssal opening, this part of the shell being quite 

 flat in that species. The ligamental area, furthermore, is of a peculiar 

 type and much narrower in B. cultrata, allowing the beaks to come into 

 close proximity. Despite the somewhat striking agreements, I am well 

 satisfied that the two species are widely distinct. In B. robusta, Miller, 

 sp., the whole anterior side is flattened, the outline different, and the 

 beaks do not curve forward as in this species, nor are they as widely 



