2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. Ill 



late Dresbachian or early Franconian age. AnconocJiihts Knight is 

 Franconian, and Chalarostrepsis new, Owenella Ulrich and Scofield, 

 and Cloudia Knight are of Trempealeauian, seemingly in that relative 

 order. These stages are Upper Cambrian. 



That each of these six genera, except Coreospira, is represented 

 by only a single known species makes it highly probable that other 

 species will be found that will extend the recorded ranges. It is 

 probable too that other genera are still below the horizon. Never- 

 theless our gradually clearing vision of Cambrian prosobranch gastro- 

 pods and their occurrence in the time-stratigraphic sequence gives 

 some basis for phyletic studies even though the results of such 

 studies are naturally tentative. 



It is to be emphasized that all but Coreospira, which is to be 

 regarded as extremely primitive if indeed it is a true bellerophont, 

 have been found only in the Upper Cambrian. The Middle and 

 Lower divisions of the Cambrian so far as we know are destitute 

 of definite prosobranchs and, of course, opistobranchs and other 

 derived groups. The asymmetrical prosobranchs, as distinct from 

 the basically symmetrical bellerophonts, do not appear in the record 

 until Trempealeauian time, the latest division of the Late Cambrian. 

 There they are represented by genera that are pleurotomarians in the 

 broadest sense such as Dirhachopea Ulrich and Bridge, Taeniospira 

 Ulrich and Bridge, and Sinuopea Ulrich, and by two peculiarly sinis- 

 tral non-pleurotomarian genera, Scaevogyra Whitfield and Matherella 

 Walcott, of which I shall write again in another place. The Lower 

 and Middle Cambrian share with later beds caplike genera, such as 

 Scenella Billings, Helcionella Grabau and Shimer, and others, which 

 are thought not to have arrived at the prosobranch stage, and the 

 anomalous group of gastropodlike forms, Pelagiella Matthew and its 

 allies, that may not be gastropods at all. 



Although all specimens of Strepsodiscus major seem to show some 

 asymmetry and always in the same sense, the species appears to be 

 quite variable in the degree that it is developed. The holotype 

 probably shows more pronounced asymmetry than any other specimen 

 in the collection. Many smaller specimens are very nearly sym- 

 metrical. The other characters are so very much those of a bellero- 

 phont and more particularly of the Cyrtolitidae that the slight lateral 

 asymmetry must be accepted, as it is in two or three other bellerophont 

 genera. However, the asymmetry in the sinistral sense occurring 

 so early in the record may be highly significant. 



There is a possibility that Strepsodiscus may prove to be congeneric 

 with Protoscaevogyra Kobayashi (1939, p. 286), in which case the 



