GASTEROPODS. 165 



day in good usage as originally proposed. Taking advantage 

 of this fact, CEhlert has recently revived Phillips' name Acro- 

 culia for the Platyceras group of shells ; but this of course 

 cannot be adopted. 



The leading characters of generic value in modern Capulus, 

 as shown by the more typical shells, as C. hungaricus Linnaeus, 

 are the obliquely conical shape, the small, often closely incurved 

 or coiled spire, the broad campanulate apertural portions, and 

 the pecular horseshoe-shaped muscular impressions. In the 

 Paleozoic forms heretofore referred to Platyceras, these fea- 

 tures have been made out most clearly in G. paralius (W. & W.) 

 and C. equilateral** ( Hall ) ; though the affinities are not less 

 striking in many other species. 



There is often considerable embarrassment in attempting 

 to separate certain Paleozoic Oapuli, on the one hand from 

 some forms of Platysto ma, -especially from those species in 

 which there is a greater or less tendency for the shells to un- 

 coil ; and on the other hand, from various genera of Patelloid 

 shells. As might be expected in a group of gasteropods pre- 

 senting so few constant characters which may be satisfactorily 

 relied upon as classificatory criteria, it is often impossible to 

 clearly distinguish between certain of these species. 



Among the first to notice the existence of Carboniferous 

 Capuli in the continental interior were Yandell and Shumard, 

 who called attention to the association of a species with an 

 Acrocrinus (afterward described by the former author as A. shu- 

 mardi}. Orlhonychia acutirostre, however, was the first species 

 of this group of gasteropods described from the Carboniferous 

 rocks of the Mississippi basin ; and was so denominated by 

 Hall in 1856. * The publication of this diagnosis was followed 

 in quick succession by definitions of other forms by Stevens, 

 Hall, Swallow, McChesney, Winchell, White and Whitfield, and 

 Meek and Worthen. 



Variation in Form. It has been noted frequently in the 

 descriptions of various Paleozoic species of Capulus, that the 

 shells often present a more or less well-defined quinquelobate 

 appearance, and that the apertural margins are for the most 



