24 GASTEROPODA. 



lying nearly or quite in tlie same plane. The whorls are 

 angulated or coronated, the aperture is polygonal, the um- 

 bilicus is very large, and there is a shelly operculum. 



The genus StixqMwlus (or Straparollus) is closely related 

 to Euomphalus, and, strictly speaking, is probably identical 



with it. It is convenient, 

 however, to retain both names, 

 employing that of Euomphalus 

 for the forms with a depressed 

 discoidal shell, with angular 

 whorls and an open umbilicus 



FiL'. 410. — Euomphalus disco rs. Upper /r. t -i rw t •^ ,^ ,•,^ 



siiui-ian, Britaiu. (fig- 410); Avhilc the title 



Straparolus may be applied 

 to those with rounded non-angulated whorls, a small um- 

 bilicus, and a more or less prominent spire. Eiiomplialop- 

 tcrus, of the Upper Silurian, includes forms allied to 

 Euomphalus, but having winged whorls, the alation being 

 perforated by canals which open internally into the cavity 

 of the shell, and externally by minute pores on tlie margin 

 of the Aving. StraparoUina, again, includes Palteozoic 

 shells, believed to stand midway between Straparolus and 

 Holopca. The shells which have been described under 

 the names of Baphistoma, Ophilcta, and Hdicotoma — prin- 

 cipally or exclusively Lower Silurian in their range — are 

 apparently closely allied to Euomphalus, if, indeed, they are 

 really separable from it. Lastly, some pahieontologists would 

 place here the singular genus Madurea, which, however, is 

 perhaps best regarded as one of the Heteropods ; and we 

 shall also temporarily consider Ophilda as belonging to the 

 same group. 



Finally, we have a group of Littorimdcc typified by the 

 genus Eissoa (fig. 411), in which the shell is small, pointed, 

 and many-whorled, with a small round aperture surrounded 

 by a continuous peristome. Many fossil species are known, 

 commencing in the Permian ; and the genus is universally 

 distributed at the present day. Among the allies of Eissoa, 

 Bissoina appears first in the Jurassic, Kcilostoma is found in 

 the Cretaceous and Eocene beds, and Diastoma and Pterostoma 

 are Eocene types. 



