84 MOLLUSCA FROM THE GREAT OOLITE. 
Trocnotoma DIscoIpEA, Roémer, sp. Plate X, figs. 10, 10a, 104, 10c. 
?Trocnus DiscorpEts, Roémer. 1836. Nordd. Oolith., p. 150, t. 11, fig. 12. 
— — D Orb. 1850. Prod. Paléont,, p. 354. 
— oo Bronn. 1848. Index Palzont., p. 1300. 
T. Testa discoided, plano-convewd, basi concavd, lato-umbilicatd; anfractibus (3) depressis, 
subconvexis, transversin lineatis, basi acutis; aperturd transverse depressd, ovata. 
(Roémer.) 
Shell discoidal, slightly convex, base concave, widely umbilicated ; whorls (3) depressed, 
rather convex, transversely lineated, lines impressed by closely-arranged longitudinal and 
very fine oblique stria ; lines upon the base acute ; aperture depressed and excavated. 
We have never been able to discover an open fissure upon this small species, but the 
general figure agrees with this genus so well that we have not ventured to assign it to any 
other. Four lines are visible upon the lower and seven upon the upper face of the last 
volution, which is angular or step-like; the first two turns are smooth and rounded : rare. 
Height 3 lines, basal diameter 7% lines. 
Locality. Minchinhampton ; Coral Rag, near Hildeshetm (Roémer). 
Stomatra, Zam. 1801. 
Sromax, Montfort. 1810. 
Shell suborbicular or oblong, generally ear-shaped and depressed ; in most species the 
spire is prominent, but not produced, nor elongated ; sometimes, however, it is very small, 
marginal, and inconspicuous. Aperture mostly longitudinal; im some species nearly 
orbicular ; in others much elongated ; always very large ; its edges entire, united, at the 
upper part, and scarcely modified or altered in form by any portion of the last volution. 
Volutions from two to four. (@. B. Sowerby.) 
From the characters of the aperture and the presence of the carina, we have 
ventured to assign the following shell to the genus Sfomatia (Lam.), from most of the 
recent species of which it differs in having a depressed spire, and the lines of growth and 
spiral striz very regular, and sharply defined. Should other specimens afford further 
generic distinctions, we would suggest the name Megastoma for it. 
