D2 EOCENE MOLLUSCA. 
deep and large depressions for the reception of the teeth 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. Alein, 1753. 
Gen. Char. Shell inequivalve, inequilateral, obliquely oval; left valve the larger 
or more tumid, right valve with a byssal sinus; cartilage-pit oblique; hinge sometimes 
edentulous, at others with one or two small cardinal teeth, 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. ‘he hammer oyster (J/a//eus), a recent shell, with an extension of 
the hinge-margin 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. 'Vhere are also some fossils of the older rocks which bear a very strong 
resemblance ; these have been elevated into genera, under the names of Pfernea, Ptero- 
nites, Pteroperna, Ambonychia, &c., each presenting some small distinctive character. 
The claim of A/eleagrina to isolation appears to rest upon a less extended hinge-margin, than 
that which, in the type of this genus (I/ytilus hirundo), 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. ‘I'he form of the shell, divested of its extended hinge-line, bears a strong 
resemblance to that of AZyéilus. 
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. 
