NATURAL SCIENCES OF PHILADELPHIA. 319 



smaller ones being a little depressed so as to form a sliallow 

 mesial sinus that is not continued to the beak. Crossing all of 

 these plications of both valves, are numerous fine lines of growth ; 

 while the entire surface, as seen under a magnifier, is occupied by 

 minute projecting points, like grains of sand; and, between these, 

 a higher magnifying power shows the whole surface to be very 

 minutel}' and regularly punctate. 



Length, 0.37 inch ; breadth, 0.50 inch ; convexity, 0.21 inch. 



Until the distinctions between the genus Befzia, and the pro- 

 posed genus Trematospira (if any exist) are better defined, and 

 the interior of the species here described can be determined, it is 

 not possible to say to which of these groups it most properly 

 belongs. 



Specifically, however, it seems to be closely allied to Trema- 

 tospira gibbosa of Hall, from tlie Hamilton group. Yet it differs, 

 not only in having two to three more plications on each side, but 

 also in having^ five instead of three a little raised to form the 

 mesial fold of the dorsal valve (the middle one being also much 

 smaller), and four depressed to form the mesial sinus (the middle 

 two being much smaller than the others). It so nearly resembles 

 the New York form, however, that I should almost be inclined to 

 suspect that it might be only a variety of the same species, if it 

 were not found at a so much lower horizon. It must be very rare, 

 as I have only heard of the single typical specimen being found. 



Locality and jJositio72. — Cincinnati group of the Lower Silurian; 

 from the basal beds at Cincinnati, Ohio. Mr. Dyer's collection. 



LAMELLIBRANCHIATA. 



AMBONYCHIA (MEGAPTERA) ALATA, Meek. 



Megapteraa Casei?^ James, 1871. Cat. Fossils of Cincmnati Group (not 

 Meek and Worthen). 



Shell attaining a moderately large size, subtrigonal in general 

 outline, compressed postero-dorsall}^, and more convex in the um- 



' In first proposing the name Megaptera, for tliese great winged species, 

 in 1866, Mr. Worthen and the writer were not aw'are that this name had 

 been previously used by Dr. Gray for a genus of Whales. Naturalists do 

 not agree in regard to the propriety of retaining the same name for different 

 genera or subgenera in such cases. Where the groups belong to the same 

 1812.] 



