PROSOBRANCHIATA. 



77 1 



Fig. 658. — Straparollus Dionysii, from 

 the Carboniferous Limestone of Bel- 

 gium. (After Zittel.) 



and a large umbilicus, the whorls being rounded and non-angulated, 

 the mouth round or oval, and the surface smooth or striated. The 

 species of this genus range from the Silurian to the Trias. Appar- 

 ently related to the preceding is Pla- 

 tyschisma, in which the shell is thin, 

 obtusely conical, ventricose, umbili- 

 cated, and smooth, " with a shallow 

 sinus in the outer lip, but with no 

 defined band" (R. Etheridge, juri.) 

 The genus appears to range from the 

 Silurian to the Carboniferous. 



The genus Euomphalus (fig. 659), 

 as defined by Lindstrom, comprises 

 discoidal shells, in which the whorls are contiguous or disjunct ; 

 the umbilicus is wide ; and the outer lip of the aperture possesses 

 on its upper side a shallow notch or sinus, which gives rise to 

 a more or less elevated revolving ridge or keel in the middle or 

 upper part of the convexity of the whorls. The mouth of the 

 shell is more or less angulated, 

 while the apex is filled up with 

 a secondary deposit of shell, 

 and the interior is often di- 

 vided off by transverse shelly 

 partitions. The species of 

 Euomphalus range from the 

 Silurian to the Carboniferous, 

 one of the most familiar types 

 being the E. pentangulatus 

 (fig. 659) of the Carbonifer- 

 ous Limestone. The well- 

 known Silurian shells which have been described under the names 

 of Euomphalus discors and E. rugosus have been shown by Lind- 

 strom to have possessed a nacreous shell, and to be referable to the 

 Turbinida. 



The name of Schizostoma is applied by De Koninck to forms 

 with a shell (fig. 660) essentially similar to that of Euomphalus, as 

 above described, but having two spiral keels, one on the upper and 

 one on the lower aspect of the convexity of the whorls. It would 

 seem that Ophileta is a synonym of Schizostoma, and in that case 

 forms of this type range from the Ordovician to the Carboniferous. 

 The forms to which the name of Ecculioi7iphalus has been applied 

 (fig. 661) would appear, again, to be essentially identical with 

 Euomphalus, from which they differ simply in the fact that the 

 shell is evolute, the whorls not being in contact. Forms of this 

 type are known to range from the Ordovician to the Carboniferous. 



Fig. 659. — Euomphalus pentangulatus, of the 

 natural size, from the Carboniferous Limestone of 

 Ireland. (After Woodward.) 



