72 MOLLUSCA FROM THE GREAT OOLTTE. 



Sub-genus, Crossostoma. 



C. Testa crassd, turbinatd, lavi, subdepressd ; anfractibus subplanis, paucis; apice 

 obtuso; aperturd subrotundd, integrd; columella dentem obtusam form ante; labio externo 

 lievi, umbilico nullo. In estate senici ajjerturd contractd crassd, orbiculari, lamina 

 testaced fabelliformi 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 Delp/iinula, and is surrounded by an additional shelly irregular lamina, which 

 projects from it in every direction. 1 



Crossostoma Prattii. Plate XI, figs. 21, 21 a. 



C. Testa parvd discoided Icevi ; spird subpland, vix elatd; Uriels incrementi rugis prope 

 aperturam sitis; aperturd parvd, orbiculatd, labris incrassatis, lamina 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 plicae 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. 



1 M. A. D'Orbigny has described a species belonging to this sub-genus, under the name Delphinula 

 reflexilabrum, from the Lias of Fontaine-Etoupefour : " Shell smooth, resembling a Turbo, but having a 

 reflected, sharp, lamellar peristome surrounding the mouth." — Prod. Paleont., p. 229. 



