1890.] NATURAL SCIENCES OF PHILADELPHIA. 151 



perhaps, nowhere existed among fossil uiollusks than in tlie group 

 under consideration. This utter hick of agreement among Avriters is 

 directly traceable to a number of causes : the majority of the species 

 have been described from very few or single specimens, with little 

 regard to tlie forms already known ; no attention whatever was paid 

 to variability and indeed the range of the latter has only very lately 

 been made out with any degree of certainty ; comparisons with in- 

 dividuals from localities more or less widely distant have been made 

 only in excei^tional cases. 



General Features. The leading characters of generic value in 

 modern Capulus, as shown by the more typical shells, as C. humjar- 

 icus Linne, are the obliquely conical shape, the small, often closely 

 incurved or coiled spire, the broad campanulate aj)ertural portions 

 and the peculiar horse-shoe-shaped muscular impressions. In the 

 paleozoic forms heretofore referred to Platyeeras these features have 

 been made out most clearly in C. paralius (AV. & W.) and C. equi- 

 latendia (Hall) ; though the affinities are not less striking in many 

 other species. 



In a group of more than three hundred described paleozoic spe- 

 cies having so few salient characters for classification and such a great 

 range of variation as the forms assigned ta Platyeeras it is hard to 

 foresee the difficulties in attempting to arrange satisfactorily the manv 

 different forms. The genus may ultimately admit of a suitable separa- 

 tion into several more or less well marked subdivisions ; and the many 

 forms make such an arrangement very desirable. It can, however, 

 only be accomplished after a careful and critical revision of the en- 

 tire group. The placing of Platyeeras, OrtJionychia, etc., as sub- 

 genera under Oipulm, as has been done by ZitteP and others, man- 

 ifestly does not meet the requirements, at least in so far as the Amer- 

 ican species are concerned. It is probable that all of the described 

 Platycei-ata cannot be included under Capulus. Just which ones, re- 

 mains for future comparisons to decide. There seems to be good 

 ground for believing that further study will show that a number of 

 the paleozoic forms in question belong more properly to genera closely 

 allied to Capulus rather than to Capulus itself. This would carry back 

 the antiquity of certain modern genera farther than has hitherto been 

 considered possible, A recent critical examination of certain des- 

 cribed Platycerata also discloses that they belong to families entirely 

 difterent from those supposed. 



1 Handbuch der Pal?eontologie, II Band, p. 210. 



