389 CRETACEOUS GASTROPODA 
The variety in the form of the shell of the palseozoic* and the secondary* species 
is indeed almost greater than it is to be met with in the Trocurpz. The only great 
difficulty which accompanies a more detailed distinction of the Plewrotomarie is 
the rarity of the outer lip being perfectly preserved ; it would be, therefore, in many 
instances perfectly useless to propose any generic distinctions upon imperfect 
specimens. The general form of the shell, the ornamentation, position of the band 
on the whorls; and the form of the slit in the outer lip are the principal characters 
upon which further generic distinctions have to be made. 
5. Ptychomphalus, Agass., 1887 (Min. Conch., Ger. edit., pp. 28, 222, ete. 
Cryptenia, Desl., 1865, Bull. Soc. Linn. Norm. IX, p. 424). 
Agassiz proposed the name for two species described by Sowerby (Min. 
Conch. II, pp. 159-160, pl. 171) as Helix ? striatus and cirriformis, being helici- 
or trochi-form shells, with an almost smooth surface, a thick callosity covering 
the umbilical region and a distinct band on the posterior portion of the whorls, 
terminating at the aperture with a short slit. Both the original species are 
paleeozoic, in which period this genus is very largely represented, being by its 
smooth surface and the umbilical callosity readily distinguished from other Plewro- 
tomarie. Some naturalists have referred these species to Umbonium (= Rotella) ; 
others to Pleurotomaria, pointing, however, to the remarkable distinctions between 
them and typical species of that genus. Eug. Deslongchamps lately proposed for 
these forms the name Cryptenia, quoting the liassic Pl. heliciformis as the type of 
the genus. Of cretaceous species I only know Pl. anomala, Pictet and Camp., 
which belongs to Plychomphalus ; of jurassic species there are about ten, and of 
palzeozoic about thirty, known. 
6. Leptomaria, Desl., 1865 (Bull. Soe. Linn. Norm. IX., p. 423). Shell sub- 
turbinate, rather thin, generally ornamented with very numerous subequal spiral 
strie, band very narrow placed near the middle of the whorls, slit in the outer 
lip very deep, axis solid or hollowed out. This is a very well marked group of 
Prevroromarup®, deserving of generic distinction. The species are, as pointed 
out by Eug. Deslongchamps, hardly found in the paleeozoic and lower secondary 
formations, the first typical forms being met with in the lower beds of the 
Dogger. About one-half of all the cretaceous Pleurotomarie belong to this 
genus, though the actual number of the species is most probably not so large as 
represented by the numbers in Catalogues, many of the so called species being 
only varieties of others. Great mischief is occasionally done by the application 
of specific names to various casts, whenever they are observed to exhibit a slight 
difference in the shape of the whorls. Such distinctions must be, however, applied 
with extreme caution, as the shape of the whorls in all Pzzvroromarips is very. 
variable. This is pre-eminently the case in Leptomaria. I may also here mention 
that there are often species of this genus met with in which irregularities in the 
growth of the band occur. I have described such a species from the alpine 
* See Patwozoic foss. of Phillips, McCoy, Carb. fossils of Koninck, Sandberger’s Schichtensystem der 
Rheinlande, d’Orbigny’s Pal. francaise, etc. 
