72 MOLLUSCA FROM THE GREAT OOLITE. 
Sub-genus, CROSSOSTOMA. 
C. Testd crassd, turbinatd, levi, subdepressd; anfractibus subplanis, paucis; apice 
obtuso; apertura subrotundd, integra; columelld dentem obtusam formante; labio externo 
levi, umbilico nullo. In «tate senici aperturd contracté crassdé, orbiculari, lamindé 
testaced flabelliformi cinctd. 
Shell thick, turbinated, smooth, somewhat depressed or Rotelliform ; whorls more or 
less flattened, few; apex of the spire obtuse, depressed; aperture nearly circular, entire ; 
the columella forms at its base a simple prominent obtuse tooth ; the outer lip is smooth ; 
there is no umbilicus. In the oldest state of growth, the aperture becomes contracted 
by the deposition of shelly matter; it is perfectly orbicular, the circumference very 
thick, and is encircled with a thin frilled appendage, always irregular, and more or less 
produced. 
In this genus the aperture undergoes a remarkable change as it approaches the last 
state of growth. The surface is very smooth, the figure Rotelliform, and the aperture is 
that of a smooth depressed Monodonta ; and this is the usual condition in which the several 
species occur. Finally, however, a few thick lines of growth closely follow each other ; the 
columella is concealed by a deposition of shelly matter; the aperture becomes precisely 
that of a Delphinula, and is surrounded by an additional shelly irregular lamina, which 
projects from it in every direction." 
Crossostoma Prati. Plate XI, figs. 21, 21a. 
C. Testa parva discoided levi; spird subpland, viv elatd; lineis incrementi rugis prope 
aperturam sitis; aperturd parvd, orbiculatd, labris incrassatis, lamind testaced abnorme 
cincto. 
Shell small, discoidal; spire nearly flat, or but slightly elevated; the last volution has 
some rugose lines of growth situated near to the aperture; aperture small, orbicular ; the 
lips incrassated, and encircled with an irregular shelly lamina. 
The shelly encircling lamina is produced by an irregular expansion of the ultimate fold 
of growth; the few rugose plicee have the more remarkable aspect, as the whole of the 
shell, excepting within the brief space of two lines from the aperture, is perfectly smooth. 
Locality. Inferior Oolite near Bath. 
We are indebted to S. P. Pratt, Esq., F.R.S., for the original specimens obtained from 
that locality. 
1M. A. D’Orbigny has described a species belonging to this sub-genus, under the name Delphinula 
refleilabrum, from the Lias of Fontaine-Etoupefour: ‘ Shell smooth, resembling a Turbo, but haying a 
reflected, sharp, lamellar peristome surrounding the mouth.’’—Prod. Paléont., p. 229. 
