ANIMALS. 197 



The reasons for restricting AUorisma to certain kinds of shells have already been 

 stated under the head of Edmondia. 



Typified with the foregoing species, the present genus embraces a number of palseo- 

 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 Panopaeas 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 gihhosa, 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 Panopcea Aldrovandi, and P. Norvegica. It 

 would thus appear, that the forms noticed constitute a transitional link between AUo- 

 risma and the normal species of Panopcea : 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. 



PJioladomya is another genus with which AUorisma 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, Miinster, 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. 



AUorisma, as now restricted, was, I believe, the first palaeozoic 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 HiateUa 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 PI. XX, fig. 5, fully illustrates 

 both the pallial and adductor muscular impressions of the genus. Looking at another 

 species elsewhere represented (PI. XVI, fig. 3), it is evident, that the pallial sinus, in its 

 varying depth, affords good grounds for specific differences ; as in AUorisma constricta, 



1 AUorisma Munsteri {Pholadomya, id.), D'Archiac and De Verneuil, possesses rather large umbones. 



2 Pholadomya, judging from tlie nacreous character of its species, appears to have its shell-tissue of the 

 same nature as that of Pandora and Lyonsia ; whereas AUorisma, 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. 



