54 PALAEONTOLOGY OP THE UPPER MISSOURI. 



be observed, differs from all of those included in the genus Eumicrotis, in being more 

 oval in outline, more compressed, more oblique, and very nearly if not quite equi- 

 valve ; while very little difference can be seen between the prominence of its right 

 and left beaks. Its most important peculiarity, however, is the total absence of 

 any traces of a byssal notch or sinus in the anterior margin of either valve. 



The typical forms of the genus Eumicrotis are, so far as known, confined to the 

 Permian rocks in Europe, and to the Permian and upper Carboniferous of America. 

 In addition to the type E. Hawni, M. & H. this genus includes E. speluncaria, 

 Schlot. (sp.), E. radiatus, Phillips (sp.), E. Garforthensis, King (sp.) ; and E. Halli 

 and E. variabilis, Swallow (sp.). 1 



A small section of this group, of which Avicula substrlata of Munster is an ex- 

 ample, seems to be mainly confined to the Jurassic rocks, though it may also be 

 represented in the Trias. These shells should probably rank as a distinct subgenus, 

 though with our present means of comparison we are not fully satisfied that this is 

 the case. 



Some Palaeontologists will insist upon referring to the genus Pteria (Avieula), all 

 such forms as those included in the groups above characterized. A moment's com- 

 parison, however, of these shells with the recent typical forms of that genus, can 

 scarcely fail to convince any skilful Conchologist that such a mingling of types is 

 totally inadmissible in our present advanced state of Natural History. 



Eumicrotis Hawni. 



(PLATE II, Fig. 5, a, b, c.) 



Monotis Hawni, MEEK & HAYDEU, Trans. Albany lust. IV, March 2d, 1858. 

 Monotis speluncaria, var. americana, SWALLOW, Trans. St. Louis Acad. Sci. 1, 184. 

 Monotus speluncaria, Shumard 1 1859, Trans. St. Louis Acad. Sci. I, 396 (non SCHLOT.). 

 Eumicrotis Hawni, (M. & H.) MEEK, Am. Jour. Sci. XXXVII, March, 1864, 216. 



Shell subcircular, or subovate ; hinge straight, equalling about half the length of the valves ; beaks sub-central, 

 short, not oblique ; ears nearly obsolete ; base rounded, antero-ventral and postero-ventral margins rounded, the 



1 Prof. King suggests that the genus including E. speluncaria probably belongs more properly to 

 the Pectenidse than to the Aviculidee, since Mr. Carpenter had found that Avicula cygnipes of Phillips, 

 supposed to be congeneric with E. speluncaria, presents the microscopic structure of the Pectenidee, 

 and not that of the Pteriidx. We think, however, that it is very improbable that these two shells can 

 be congeneric, for we find our E. Hawni, which is even specifically very closely allied to, and certainly 

 congeneric with, E. speluncaria, shows very distinctly, under a high magnifying power, the prismatic 

 structure of the Pteriidae. The annexed cut, No. 1, represents the structure of this species as seen 



No. 1. No. 2. 



Shell structure of E. Hawni. Shell structure of E. carte. 



by transmitted light, when magnified about 300 diameters. We also observed the same Structure in 

 the Jurassic species Eumicrotis curta, Hall (sp.), as may be seen by the annexed cut, No. 2. 



In regard to the relations of Avicula cygnipes, of Phillips, to Monotis salinaria, Bronn, we would 

 merely state that we differ widely from those who would place two such shells in the same genus. 



