ACEPHALA TESTACEA. 
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adhere to all sorts of bodies, [and their form is generally modified by the surface of the objects on which 
they grow]. 
M. de Lamarck separates from the Spondylus his Plicatula, from having no external area, or disk, between the ( 
beaks ; and flat, almost equal, irregular, plaited and scaly valves, as in many Oysters. [Sp. pUcattis, Gmel., is the 
type.] 
Malleus, Lam. — 
Has a simple fossa for the ligament, as in Ostrea, with which genus Linnaeus left this one, and the more 
so as the shell is also inequivalve and irregular, but it is distinguished by an emargination on the side 
of the ligament for the passage of a byssus. 
The best known species {Ostrea malleus, Linn.), a rare and dear shell, has the two sides of the hinge extended 
so as to form something like the head of a hammer, while the valves, elongated in a transverse direction, represent 
the handle. It inhabits the Archipelago of India. Other species, which are, perhaps, but the young of the Malleus, 
have no hammer-head, and these we must be careful not to confound with the Vulsellae. 
Vulsella, Lam. — 
Has in the hinge, on each side, a little lamina projecting inwards, and it is from one of these laminse 
that the ligament, similar in other respects to that of the Oyster, is stretched to the other. On the 
side of the lamina is a sinus for the egress of the byssus. The shell is elongated in a direction perpen- 
dicular to the hinge. The species best known inhabits the Indian Ocean. 
Perna, Brug. — 
Has across the hinge several parallel fossae opposed to each other in the two valves, and lodging as many 
elastic ligaments : their shell is irregular and foliated, like the Oysters, and has on the anterior side, 
underneath the hinge, an emargination, through which the byssus passes. Linnaeus left them also 
among his Ostreae. [The recent species are brought from the Indian Ocean, and from New Holland,] 
There has been recently separated from Perna, the Crenatulce, Lam., which, instead of transverse fossae on a 
broad hinge, have little oval ones quite on the margin, where they occupy little breadth. It does not appear that 
there is any byssus. We find them often buried in sponges. To the Pernae, it is supposed, we must approximate 
some fossils which have more or less numerous fossae in the hinge answering to one another, and appearing also 
to have given attatchment to ligaments. Thus the Gervillice, Defr., have a shell almost similar to Vulsella, but 
with a hinge in some degree double ; the exterior with opposed fossae receiving as many ligaments, and the interior 
garnished with very oblique teeth on each valve. We find the casts of them with Ammonites in compact limestone, 
[Many species have occurred at various geological periods from the lias upward, to the baculite limestone of Nor- 
mandy.] The Inoeeramus, Sower., is remarkable for the elevation and inequality of the valves, of which the 
summit is hooked near the hinge, and whose texture is lamellated. The Catilles, Brongn., have, independently of 
fossae, for the ligament, a conical furrow drawn in a varix, which is bent at a right angle to form one of the margins 
of the shell. The valves are nearly equal, and of a fibi'ous texture. They appear to have had a byssus. The Pul- 
vmites, Defr., have a triangular regular shell, and its fossae, few in number, diverge within from the summit. 
Their casts are found in chalk. 
The second subdivision of the Ostracea, as weU as almost all the bivalves which follow, besides the 
single transverse [or adductor] muscle of the preceding genera, have another muscle going from one 
valve to the other, and placed in front of the mouth. It is apparently in this subdivision that we must 
place 
[The Mulleria, De Fer., — 
One of the most singular and rare of knowm genera. It is remarkable as being intermediate in its 
structure between Altberia and Ostrea, and as apparently connecting the regular freshwater bivalves 
with the irregular marine bivalves (Ostrese), and with the genus Altheria, inasmuch as in the sinus at 
the posterior extremity of the ligament it resembles the Naiades and the ^Etherise ; and in its single 
muscular impression, as well as its general form, it approaches to Ostrea.] 
Etheri^, Lam. — 
Are large inequivalved shells, as, or even more, irregular than the Oysters, without teeth to the hinge, 
and where the ligament, in part external, exists also interiorly. They differ from the Ostreae in having 
two muscular impressions. It is not ascertained that their animal produces a byssus. They have lately 
been discovered in the Upper Nile. 
Avicula, Brug. — 
Has a shell with equal valves, and a rectilinear hinge, often extended into wings on each side, furnished 
with a narrow, elongated ligament, and sometimes with small denticulations on that side which is next 
