198 GASTEBOPODS. 



Genus Naticopsis McCoy. 



The described species of Naticidae from the American 

 Paleozoic rocks number about three score. These have com- 

 monly been referred to the genera Naticopsis, Platystoma and 

 Strophostylus. The first of these three terms was proposed 

 in 1844 by McCoy, and included shells which had previously 

 been assigned to the modern genus Natica. Seven species 

 were enumerated under the new title ; but of the accompany- 

 ing figures only two showed the apertural characters. In 

 America the shells of this group were first recognized by Nor- 

 wood and Pratten, who described from the Coal Measures 

 Natica ventricosa. Shortly afterward several other forms were 

 detected and placed under the same genus. It was then found 

 that McCoy's generic term was applicable to the American 

 forms hitherto regarded as Naticre ; and still more recently 

 it was discovered that in addition to the species generally 

 recognized as belonging to Naticopsis, the genus should also 

 include several other forms now known under other generic 

 titles. 



The species that have been referred to Naticopsis appear 

 to form at least two, and possibly three, more or less well- 

 defined groups. These sections differ so greatly in several 

 important particulars that actually they should be regarded as 

 generically distinct. Meek and Worthen, recognizing the fact, 

 proposed to establish three subgenera; but their subdivisions 

 were based upon surface ornamentation. The three sections 

 were: Naticopsis proper, a group typified by JVerita subcos- 

 tata Goldfuss, but not named; and Trachydomia, including 

 N. nodosa, M. & W., N. fiollidayi M. & W. and Littorina 

 wheeleri Swallow ; besides two European species. In Nati- 

 copsis proper, as represented by the typical forms, and by the 

 majority of American species referred to the genus, the shells 

 are relatively thin ; the spire very short ; the outer lip ex* 

 tremely thin and sharp ; the inner lip also thin and slightly 

 depressed ; the last volutions generally more or less flattened 

 or concave on the upper half, and marked toward the suture 



