MOLLUSCA. ' @& 
muscles, and although the same character is possessed by most of the 
Elatobranchia, a large portion of them have but one, so that the whole are 
divided by this character into two sections, thus: 
§ Monomyaria, with one adductor muscle. 
§$ Dimyaria, with two adductor muscles. 
This character, although of much importance, is not applicable in every 
case, because a classification that would adhere to it strictly, would 
sometimes unite dissimilar groups and separate allied ones, as far as the 
other characters are concerned. The first section includes the five 
families: 1. Anomiide; 2. Placunide; 3. Ostreide; 4. Pectinide; 5. 
Aviculidee. 
Fam.1. Anomude. The genus Anomia has a principal central adductor 
muscle, but besides this it has two approximate, one passing towards the 
perforation at the apex of the smaller (and in position the lower) valve, 
where it is attached to a shelly plug, shaped like a short, flat-headed rivet, 
and inserted in the aperture, its exterior flattened surface being attached to 
extraneous bodies. The ovary terminates between the membranes of the 
mantle upon the right side, where the eggs are stored before they are 
ejected, an anomaly presented by no other ees but which, with some 
other characters, indicates an affinity with pace On this account 
Deshayes thinks that this family forms a distinct group, which should stand 
between the Brachiopoda and the Conchifera. In Anomia the margins of 
the mantle are entirely disunited ; the shell is very irregular, orbicular, the 
free or larger valve convex, and the lower one concave or flat, and taking 
the form of extraneous bodies, to which it is attached. This causes the 
shell to offer many varieties, and the species to be of difficult determination. 
The aperture in the perforated valve is only a deep sinus, narrowed at the 
margin, with the shell projecting upon one side, and forming an ossiculum 
with a flattened surface of articulation, which enters and is attached to the 
coneavity of the opposite valve by an internal ligament. This family is 
allied to the next by Broderip’s genus, Placunanomia, which has a notch in 
the side of the lower valve, presenting an analogy a that in Pedum, a 
genus of Pectinide. 
Fam. 2. Placunide. This family is represented by the genus Placuna, 
the shell of which (the only part known) is allied to Anomia and Gatien 
It is imperforate, free, inclined to circular, thin, and foliaceous in texture, 
translucent, and the valves are so flat that but little room is left for the 
animal, which must consequently be very thin. In one valve there is a V- 
shaped tooth, and in the other a corresponding depression ; ligament marginal. 
This and the preceding family are by some included in the Ostreide. 
Fam. 3. Ostreide. The genus Ostrea (pl. 76, fig. 25, O. cristigalli), 
well known as an article of food, is widely distributed in various parts of 
the world, and has been in request from a remote period. The shell is 
inequivalve and irregular, the ligament partly internal and partly external, 
and the left and larger valve is usually attached to extraneous bodies. On 
the western coast of Africa, where the branches of certain aquatic trees 
hang in the water at high tide, the oysters affix themselves to these 
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