54 CRETACEOUS GASTROPODA 
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Only by some such suppositions are we able to explain the variations noted 
above, which indeed not unusually occur in the process of growth of one and the 
same species. 
It has been proposed by several conchologists to unite the genera Straparolus 
(Euomphalus) and Solariwm in one. When, however, we compare the large 
number of species of both these genera, it appears, that the smooth or at least 
less ornamented surface of the shell, the constant want of a distinctly crenulated 
margin round the unbilicus, combined with the roundish form of the whorls of Strapa- 
rolus, makes its separation from Solariwn very desirable. Of many of the 
palseozoic Straparoli the opercula* are known, and they very much resemble those of 
Torinia, being thick and composed of numerous lamellar volutions. 
Waclurea, Emmons, or Waclurites, Lessueur, can scarcely be separated 
from the smooth paleeozoic Straparoli. The same is the case with Schizostoma, 
Brown, of which the author ealls Strap. (Huomphalus) catillus, Sow., the type. 
It is a discoid form, which no doubt unites several characters of Discohelix 
with those of Straparolus, butas the strize of growth are not bent backwards on the 
periphery, it ought to remain in the latter genus. Rémer, in the third edition of 
Bronn’s Leth. geognostica, Vol. I, p. 456, gives up the name Sehizostoma, and 
again places the above-mentioned species in Huomphalus. Deshayes, (Anim. s. 
vert. Foss., Paris, 2nd edit., Vol. II, p. 678,) seems,—for some particular reason, 
which he does not specify,—inclined to retain Schizostoma, but he states at the same 
time, that he examined a complete specimen of Strap. catillus, which exhibited 
scarcely any characters different from his Bifrontia! We shall immediately enter 
into the characters of this genus, and state in which sense it may be retained. 
It is not likely, that S?. catillus is a Bifrontia, nor yet the two species, lately added 
by Deshayes to this genus, Bif. ammonoides and Deshayesi. 
From what we have above stated regarding the characters of Straparolus, it is 
evident, that Platychisma, Conrad, cannot be, strictly speaking, looked upon as 
anything else than a smooth Straparolus with a smaller umbilicus and somewhat 
flattened volutions, but on account of the different form of these, it may be consi- 
dered as a subgeneric division. 
The largest number of S¢raparolus (Euomphalus, olim.), occur in paleozoic strata, becoming 
gradually less numerous in the subsequent formations. Of living species we actually know nothing. 
The cretaceous species will amount to about ten. Pictet (Mat. Pal. Suisse., 3me. Ser., p. 557), after 
having suggested the separation of Solarium and Straparolus, quotes of the latter six European 
species, of which two, Strap. Michaillensis, P. and Camp., and St. Moutonianus, D’Orb., are 
ornamented with strong transverse ribs. Very similar forms are known as Delphinula and 
Liotia, and it is doubtful, whether they do not more correctly belong to those groups of shells. 
Prof. Reuss named a species from the Gosau formation, Huomphalus canaliculatus, (Denksch. Akad., 
Wien, 1854, Vol. VII, p. 149, pl. 29, fig. 7). He speaks of a sharp keel on the edge of the umbi- 
licus, and as the shell resembles in every other respect Solar:wm, it requires rather better materials to 
prove, that the species is really a Straparolus. I never met with the species myself. 
* Thad lately the opportunity of examining some very fine specimens of these in Mr. Barrande’s exten- 
sive collections at Prag. 
