156 GASTBROPODS. 



group seemingly quite distinct. But later inquiry has indicated 

 that the alleged generic distinctions are actually more apparent 

 than real ; and that the two sections can, with great propriety, 

 be considered under a single term. Some recent writers have 

 even proposed to make the two genera in question identical 

 with Solarium, established by Lamarck for a group of modern 

 gasteropods. But it does not appear feasible, nor advisable, to 

 extend the limits of the Lamarckian genus, as they suggested ; 

 while practically the separation of the recent and ancient forms 

 is not difficult, and, as a matter of fact, is very convenient to 

 the systematist. 



Straparollus, as defined by Montfort, has for its type 8. 

 dionysii Mont. a form with the spire somewhat elevated, the 

 umbilicus broad and shallow, and the whorls regularly rounded. 

 Euomphalus of Sowerby, represented by E. pentanguiaris Sow., 

 includes planorbiform shells, having more or less distinctly 

 angulated volutions. With the types alone under considera- 

 tion, the two groups might appear sufficiently well marked to 

 warrant their generic separation. A more extended compari- 

 son, however, of the described species reveals no reliable 

 criteria by which the two groups may be distinguished. A 

 further consideration of these resemblances and differences of 

 divers individuals shows that they are so variable, and that the 

 gradations are so complete, that the generic limitations hereto- 

 fore usually assigned are clearly untenable. 



Briefly stated, the general characters of Straparollus are : 

 Shell rather thick, planorbiform, or depressed conical, broadly 

 and often deeply umbilicated ; whorls angular or rounded, usu- 

 ally closely coiled, but often barely in contact ; aperture sharply 

 pentagonal to sub-circular; labrum generally sharp. The sur- 

 face of the volutions is for the most part smooth, or showing 

 only numerous lines of growth ; but sometimes with one or 

 more distinct longitudinal carinse. 



In the majority of cases the carinse or angular prominences 

 on the whorls of certain Straparolli appear to be simply thick- 

 enings of the shell at those points. The internal transverse 

 section is circular, as shown when the shell is removed from 



