12 
Allorisma, n. gen. Having satisfied myself that this genus, 
as I first described it,* comprises two distinct genera, one of 
which is the Edmondia of de Koninck, 1 will here briefly 
point out a few of its distinctive characters, as now restricted, 
reserving a fuller description for my Monograph. I consider 
Allorisma regularise as the type of the genus. Allorisma 
possesses a siphonal inflexion, an edentulous hinge, and an 
external cartilage. Tt differs from Pholodomya — a closely 
related genus—in the want of ribs proceeding from the um- 
bones to the ventral margins, the valves being more or 
less wrinkled transversely. The anterior muscular impres¬ 
sions have a low or proximo-ventral position as in Thracia 
pubescens. 
Allorisma elegarts, n. sp. Form very inequilateral: both 
ends closed; anterior one the shortest, and oblique superiorly ; 
posterior one squarish: umbones somewhat gibbous: dorsal 
slopes with a faint angle running from the umbone to the pos¬ 
terior end of the shell: surface slightly wrinkled transversely, 
and crowded with minute pimples: pallial sinus shallowish. 
Amphidesma lunulata, Keyserling, of the Petchora Permien 
marls, may be the same species. Humbleton and Whitley. 
Psammobia ? subpapyracea, n. sp. A thin, smooth, ellipti¬ 
cal species. Length one third of its width : umbones slightly 
prominent, one third of the width of the valves from the ante¬ 
rior end : terminations rounded. Humbleton. 
Chiton Loftusianus , n. sp. A provisional description of this 
species has already been given in the “London Geological 
Journal,” vol. I. I embrace the present opportunity of nam¬ 
ing it after W. K. Loftus, to whom 1 was first indebted for the 
idea that it belonged to the genus Chiton. Tunstall Hill. 
Macrocheilus symmetricus, n. sp. Fusiform: smooth : whorls 
slightly rounded : mouth oval, more rounded in front than be¬ 
hind, and a little more than a third of the shell in length. 
* Annals of Natural History, November, 1844. 
f Geol. Russ., vol. 2, pi. 19, fig. 9.—The fossil under this name, in 
pi. 21, fig. 11, is an Edmondia. 
