ANIMALS. 197 
The reasons for restricting A//orisma to certain kinds of shells have already been 
stated under the head of Hdmondia. 
Typified with the foregoing species, the present genus embraces a number of palzeo- 
zoic forms, which are edentulous, oblong transversely, more or less wrinkled in the 
same direction ; possess small umbones,! and an external cartilage ; have a granulated 
exterior, and the anterior adductor muscular impressions proximo-ventrally situated. 
It closely agrees with certain secondary Panopzas in form, and in being granulated 
externally. There is also a further agreement ; inasmuch as in some of these shells, for 
example, the so-called Lutraria gibbosa, Sow., and L. decurtata, Goldfuss, the teeth are 
little more than rudimentary, or mere flexuous expansions of the cardinal margins,—not 
strong conical projections, as in the recent Panopea Aldrovandi, and P. Norvegica. It 
would thus appear, that the forms noticed constitute a transitional lmk between A//o- 
risma and the normal species of Panopea: whether these transitional forms ought to be 
considered as constituting a distinct generic group, which I have an impression has 
been named Platymya by Agassiz, is a point on which I am not at present prepared to 
enter. 
Pholadomya is another genus with which A//orisma is intimately related ; it differs, 
however, in having a number of ribs passing posterio-obliquely from the umbone to 
the ventral margin of the valves; in being, in general, widely gaping at the ends; 
and, perhaps, in the nature of its shell-tissue.” As regards hinge-characters, both 
genera are in accordance with each other, that is, they are divested of teeth, and have 
an external cartilage. 
The two co-ordinate and related groups Lysianassa, Minster, and Cercomya, 
Agassiz, require only a passing allusion ; as the former with its V-shaped ribs, and the 
latter with its attenuated posterior extremity, are clearly distinct genera. 
Allorisma, as now restricted, was, I believe, the first paleeozoic genus made known, 
as possessing a sinus in the pallial impression. Shortly before I published my first 
paper on the genus, I had seen this character faintly displayed on a specimen of the 
so-called Hiatella sulcata, Fleming, in Mr. Tate’s collection; and since then I have 
repeatedly collected specimens of the same shell in Redesdale, with it displayed in the 
most satisfactory manner. ‘The specimen represented in Pl. XX, fig. 5, fully illustrates 
both the pallial and adductor muscular impressions of the genus. Looking at another 
species elsewhere represented (Pl. XVI, fig. 3), it is evident, that the pallial sinus, in its 
varying depth, affords good grounds for specific differences; as in dl/orisma constricta, 
' Allorisma Munster (Pholadomya, id.), D’Archiac and De Verneuil, possesses rather large umbones. 
2 Pholadomya, judging from the nacreous character of its species, appears to have its shell-tissue of the 
same nature as that of Pandora and Lyonsia; whereas Allorisma, from its granular surface, has appa- 
rently a tissue agreeing with that of Thracia. The distinctive characters of the histology of Pandora and 
Thracia have been published by Dr. Carpenter in his 2d report ‘ On the Microscopic Structure of Shells,’ 
in the British Association Report for 1847. 
