20 PALEONTOLOGY OF THE UPPER MISSOURI. 



usual type of Spirifer as generally understood), that we cannot see how the 

 can be separated more than as sections or subgenera. 



It will therefore be seen, that in our present state of knowledge, -it seems to be 

 most natural to exclude entirely from the genus Spirifer, all the punctate species. 

 Entertaining these views, it becomes necessary for us to explain why we have placed 

 Ambocoelia, which has been described as " fibrous (or fibro-punctate)" with " lustre 

 pearly," as a synonym of Martinia, one of the sections of Spvrif&r. 



Our reasons are, in the first place, that we see nothing in the form and external 

 appearances of the type of this proposed genus, to separate it from Martinia ; 

 especially since the closely allied Amboccelia gemmula, of McChesney, which was 

 also included by the author of Amboccelia in that group, sometimes has its dorsal 

 valve a little convex indeed occasionally as much so as the scarcely distinguishable 

 European Spirifer Urei and S. Clanyanus though it is more frequently flat or 

 concave as in the type of Ambocoelia. Again, we know from an examination of 

 New York specimens of the type of Ambocoelia, in the collection 'of Mr. Worth en, 

 State Geologist of Illinois, that it has internal spires arranged as in Spirifer. 

 Thinking, however, that the punctate structure, and the supposed pearly lustre 

 of this type, might warrant its separation, we subjected authentic specimens of it 

 from the Hamilton Group, New York, to a careful microscopical examination, both 

 in polished sections, and in thin broken fragments, and although we could distinctly 

 see the usual fibrous structure so generally characteristic of the Brachiopoda, we 

 failed to detect any traces of perforations, even by the aid of a high magnifying power. 



In regard to the lustre, we think the word "pearly" must have been inadvertently 

 written, or wrongly printed, for although fractured surfaces of this shell present a 

 shining, somewhat silvery appearance, not unusual in the shells of fossil Brachiopoda, 

 it cannot be said to be pearly, as that term is usually understood by Conchologists. 



From these facts we do not feel prepared to admit Ambocoelia even as a distinct 

 section from Martinia, until some more reliable differences can be pointed out. 



The genus Spirifer, as here characterized, commenced its existence during the 

 Lower Silurian epoch, and ranges through the more modern formations into the 

 Triassic rocks. 



Spirifer (Martinia) pla no-con vexns. 



Spirifer plano-convexus, SHUMARD, Report Geol. Survey Missouri, 1855, part Palieontology, 202. 



Amboccelia gemmula, McCnEbNEY, New Palaeozoic Fossils, 1860, 41. 



Amboccelia yfmmula, HALL, Thirteenth Report Regents University, N. Y. 1880, 71. 



Comp. Sp. Urei, FLEMING, Brit. Animals, 1828, 376. 



Shell small, plano-convex or concavo-convex, young individuals usually longer than wide, adults wider than 

 long ; hinge line always shorter than the greatest transverse diameter of the valves ; lateral margins and front 

 rounded ; surface apparently smooth excepting a few concentric marks of growth but when examined with a 

 magnifier, it is sometimes seen to be beset with the bases of minute hair-like spimiles. Dorsal or smaller valve 

 slightly convex near the beak, but usually concave around the front and antero-lateral margins, especially in 

 adult individuals ; trnnciito-orbicular in outline, usually very faintly depressed at the middle of the front ; beak 

 very small, not incurved, nor projecting beyond the cardinal margin ; area about half as wide as in the other 

 valve, flat, and standing nearly at right angles to the plane of the valve ; foramen extending to the beak. Ventral 

 valve strongly convex, sometimes very faintly flattened along the middle, but without a mesial sinus ; beak very 

 prominent, gibbous, and distinctly arched back over the hinge ; area moderate, triangular, arched, well defined, 

 and generally longer on the hinge side than the lateral margins ; foramen usually higher than wide, provided with 

 slightly raised lateral margins, not closed (so far as known) by a pseudo-deltidium. (Muscular impressions 

 unknown.) Spiral appendages each consisting of six or seven distant turns. 



