INVERTEBRATE PALAEONTOLOGY. 25 



The genus Chlamys, however, as here understood, mighl be conveniently 

 divided into two or more subgenera, though no attempt will be made in (his 

 direction by the writer at present. In using the name Chlamys for a section 

 of this group, under the generic name Pecten, Dr. Stoliczka takes P. bifrons, 

 Lam., as the type of the former, and expressly excludes P. Island/cits, Linn.; 

 but as (his shell was Bolten's first species of Chlamys, and especially as it 

 had been cited by Herrmannsen and others as its type, there seems to be no 

 authority tor considering any other form as such. 



The group Chlamys, with the limits here assigned it, seems to be 

 entirely unknown in the Palaeozoic rocks; the similar species of that age, 

 often described under the name Pecten, belonging to the genera Aviculo- 

 pecten, Euchondria, Pernopecten, Entolium, and probably to several unde- 

 scribed genera. In the Coal-Measures of Illinois, there is a shell having 

 very much the external appearance of some of the less strongly costate forms 

 of Chlamys, but its cartilage-pit is conical and quite oblique, more like that 

 of souk 1 types of the Pteriidce (— Aviculidaf). 



It is probable, however, that this genus was represented as far back as 

 during tbe.Triassic epoch, and it seems to range through all subsequent 

 geological formations, increasing in the number of species to the present 

 time.* 



Clilamys Ncbrasccnsis, M. & H. 



Plate 10, figs. 6, a, b, c. 

 Pecten Nebrascensis, Meek anil Itayden (1856), Proceed. Acad. Nat. Sci. Philad., 87. 



Shell small, suborbicular, rather inequivalve, the left valve being some- 

 what more convex; ventral margin regularly rounded, and about semicircular 

 in outline; anterior and posterior margins converging from near or above the 

 middle toward the beaks; hinge generally less than the breadth of the 

 valves; ears more or less nearly equal, distinctly compressed, both separated 

 from the margin below by somewhat angular sinuses, that under the anterior 

 one of the right valve being deeper than the other; surface of each valve 

 ornamented by about twelve to fifteen comparatively large, strongly elevated, 

 usually simple, angular, radiating plications, separated by generally slightly 



* I am far from agreeing with Dr. Stoliczka that tins genus, as here restricted, "occurs in all 

 formations, from the oldest upward." It would at least, puzzle any one to find a Silurian or Devonian 

 shell of this group. 



I II 



