80 PALAEONTOLOGY OF THE UPPER MISSOURI. 



terior cannot be seen. They can be readily distinguished, however, Avhen wc have 

 an opportunity to examine the hinge, which in Pterinia differs from that of Pteria, 

 in the possession of a more or less broad cardinal facet, marked with linear cartilage 

 furrows, but without a cartilage pit ; as well as in having obUque posterior and 

 anterior teeth not found in the true Pterias. From Meleagrina and McdJceus, which 

 some authors include in this genus, the Pterias can be readily distinguished by 

 obvious differences of form. 



It will be very difficult to determine at what particular period this genus, as 

 properly restricted, was first introduced, imtil the nature of the hinge of many 

 extinct species can be determined. Palaeontologists, Avho are often too much in- 

 clined to lose sight entirely of the existing types upon which so many genera of 

 MoUusks were originally founded, refer to it many species from the Palaeozoic 

 rocks ; but many of these older species are known to be true Pterinias ; and it is 

 more than probable, as already stated in the remarks on this family, that the 

 Silurian, Devonian, and many, if not all, of the Carboniferous and Permian species 

 referred to Avicula, wiU be found to belong to Pterinia, BakereUia, &c., or to 

 undescribed genera. Our present impression is, that typical species of Pteria did 

 not exist previous to the Cretaceous epoch, and that probably none of the forms from 

 rocks older than the Jurassic, or possibly from the Trias, can be properly included, 

 even as distinct subgenera. The genus is represented in the Cretaceous and Tertiary 

 deposits, but seems not to have been more extensively developed at any past time 

 than at present in our existing seas. The living species are found on the coast of 

 South America, of the British Islands, and in the Mediterranean and Red Seas, 

 the Indian Ocean, &c. 



Subgenus OXYTOMA. 

 Pteria ITIunsteri. 



Avicula Munsteri, Bronn, Leoh. Zeitsoh. 1829, 76. 



Avicula Munsteri, GoLDF. Petref. Germ. II, 131, cxviii, a, b, c, d, e,f, g, h, 



Monotis Munsteri, Quenstedt, Wurtt. 341; Is. Der Jura, II, 1856, 440, Ix, 6. 



Shell obliquely oval ; hinge line less than the greatest parallel diameter of the valves, and ranging at an angle 

 ^ ^ of 550 to 6OO above the oblique longer axis of the shell ; anterior margin slop- 



ing with a graceful backward ctirve into the rather narrowly rounded postero- 

 basal extremity ; posterior margin ascending forward nearly parallel with the 

 anterior outline to the wing, where it curves rather abruptly backward so as to 

 form a distinct rounded sinus. Left valve rather gibbous ; beak convex, and 

 projecting slightly beyond the hinge margin ; posterior ear flattened, subtrigonal, 

 and terminating behind in a mucronate angle, but not extending so far back 

 as the postero-basal margin ; anterior ear small, rather convex — form unknown. 

 Surface (left valve) ornamented by about ten to fifteen moderately distinct 

 slender radiating costse, separated by spaces four to six times their own breadth. 

 At the middle of each of these spaces there is usually a smaller rib, which 

 dies out before reaching the umbo ; and between each of these and the prin- 

 cipal ribs, still smaller radiating striae are seen — the whole being crossed by 



a few small marks of growth, and (probably on well preserved specimens) concentric striae. 



Diameter at right angles to the hinge, about 0.93 inch ; do. parallel to the same, about 1 inch ; convexity of left 



valve, near 0.24 incli. 



We have referred this shell (provisionally) to Avicula Munsteri, of Bronn, rather 

 because the imperfect specimens we have yet seen do not exhibit any reliable 



Pteria Munsteri. 



A. Outside view of left valve. 



B. Profile of same. 



