52 EOCENE MOLLUSCA. 



dee]) and large depressions for the reception of the teetli of the opposite one. Auricles 

 small. The muscle-mark is large, rounded, and eccentric. This species strongly resembles 

 Sp. Cisalpinus, Brongniart, and may probably (as suggested by M. Deshayes) be only a 

 strongly marked variety of that shell. 



AVICULA. Klein, 1753. 



Gen. Char. Shell inequivalve, inequilateral, obliquely oval ; left valve the larger 

 or more tumid, right valve with abyssal sinus; cartilage-pit oblique; hinge sometimes 

 edentulous, at others with one or two small cardinal teetli, and an elongated lateral 

 one; hinge-line rectilinear, with the extremity generally prolonged ; muscular im- 

 pressions large, subcentral ; pedal scar high in the umbonal region ; impression of the 

 mantle entire ; connexus ligamentous. 



Animal obliquely triangular; the edges of the mantle disunited, except at one point 

 where the juncture separates the incoming from the outgoing canals; margins fringed, and 

 furnished with a pendant curtain ; foot small, subcylindrical, or digitiform, grooved ; byssus 

 sometimes solid, with an expanded termination. 



There is much difficulty in assigning a proper limit to this genus, which has so many 

 near relations. Conchologists are greatly at variance as to what should be included within 

 its generic boundaries. The hammer oyster {Malleus), a recent shell, with an extension of 

 the hin^e-maroin on each side of the umbo, has been considered as not entitled to generic 

 distinction ; the young shell being extended only on one side, and it is then very like a 

 true Avicula. There are also some fossils of the older rocks which bear a very strong 

 resemblance ; these have been elevated into genera, under the names of Pterinea, Ptero- 

 nites, Pleroperna, AiiibonycJiia, &c, each presenting some small distinctive character. 

 The claim of Meleagrina to isolation appears to rest upon a less extended hinge-margin, than 

 that which, in the type of this genus {j\Ji/til//sInrnndo), gives such a winged-like form to that 

 shell ; this appendage is exceedingly variable in different species, and indeed is of different 

 lengths in the individuals of the same species, the young differing from the parent shell ; 

 while also among the full-grown specimens, this character is by no means permanent in 

 the same species. The form of the shell, divested of its extended hinge-line, bears a strono- 

 resemblance to that of Mytilus. 



The connexus in this genus is in general simple, and spread over a large external area. 

 Twelve or thirteen species have been described from the French Eocene deposits, and 

 a few from the more recent formations. 



The only species of this genus now found on our own coasts and in the Mediterranean 

 ranges, in the latter sea, according to Mr. M'Andrew, from eight to thirty-five fathoms. 



