100 PALAEONTOLOGY OF THE UPPER MISSOURI. 



fossil species in the same way; thus he ranged the fossil shells he supposed to 

 belong to the genera Pecten, Donax, Unio, Buctinum, &c., under the names Pec- 

 tenites, Donacites, Unionites, Succinites, &c. Hence we cannot accept any of these 

 as generic names established by him, when he may have by mistake included types 

 of undescribed genera. 



The first author, after Schlotheim, so far as our knowledge extends, who used the 

 name Myacites, was Bronn (Leth. 1837); but as he used it in much the same sense 

 that Schlotheim did that is, as a provisional receptacle for fossil shells supposed 

 to belong to the genus Mya, as he did Turbinites, &c. we can scarcely regard him 

 as having established it as a genus. In 1840, however, Munster adopted it regu- 

 larly as the name of a distinct genus, and described under it Myacites musculoides, 

 M. ventricosus, M. elongatus, M. radiatus, M. mactroides, Schlot. ; M. radiatus, M. 

 grandis, M. obtusus, Munster ; and M. Albertii, Voltz ; all of which appear to be 

 congeneric with the first or typical species, with possibly one or two exceptions. 

 We regard it as an established genus from that date, with M. musculoides as its type. 



As already stated in the remarks respecting the affinities of the genus Allorisma 

 (page 37), this group, as we understand it, seems to be very closely related to the 

 Permian and Carboniferous shells upon which that genus was founded. For a 

 statement of the principal points of difference between these two groups we would 

 refer the reader to the remarks on page 37, in connection with the description of 

 Allorisma. 



These shells are also related to the genus Pholadomya, from which they differ in 

 never having radiating costse, as well as in their granulated surface. They also 

 seem to be entirely destitute of pearly internal layer seen in the shells of that 

 genus. From Panopcea, to which they are often referred, they differ in being much 

 thinner shells, as well as in their granulated surface and much more faintly marked 

 muscular and pallial impressions. Indeed, they appear, from all analogy, to belong 

 even to the very distinct family Anatinidce, instead of the Saxicavidce. 



The genus Myacites probably first appeared during the Triassic epoch, though we 

 are not sure that some of the species usually referred to Allorisma from the older 

 rocks are reaUy generically distinct. It seems to have reached its maximum de- 

 velopment during the deposition of the Jurassic rocks, and continued to exist until 

 after the commencement of the Cretaceous epoch, during the earlier part of which 

 it probably became extinct. 



JUyacites nebrascensis. 



(PLATE IV, Fig. 5.) 

 Myacites Nebrascensis, MEEK & HAYDEN, Proceed. Aoad. Nat. Sci. Phila. May, 1860, 182 ; ib. Oct. 1860, 418. 



Shell elongate-snbelliptieal, rather convex ; extremities narrowly rounded, the posterior end being sometimes 

 apparently obliquely subtruncate, and more or less gaping above. Base nearly straight, or very slightly sinuous, 

 along the middle ; rounding up gradually toward the ends. Dorsum behind the beaks, concave in outline ; poste- 

 rior umbonal slopes gibbous, or prominently rounded ; antero-ventral region a little compressed, or contracted, from 

 near the middle of the base, obliquely forward and upward. Beaks moderately elevated, gibbous, incurved, and 

 located near the anterior end. Surface ornamented by concentric striae, and small, very obscure, irregular parallel 

 wrinkles. 



Length, about 1.43 inch ; height, 0.69 inch ; breadth, 0.59. 



Our specimen being a cast, has probably lost some of its finer surface characters, 



