MOLLUSCA. 61 



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 : 



§ Monoinyaria^ with one adductor muscle. 

 §§ Dhnyaria^ 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. Anomiidse ; 2. Placunidse ; 3. Ostreidae ; 4. Pectinidae ; 5. 

 Aviculidae. 



Fam. 1. AnomiidoB. 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 slielly 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 conchifer, but which, with some 

 other characters, indicates an aftinity with Brachiopoda. 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, tlie 

 free or larger valve convex, and the lower one concave or flat, and taking 

 the form of extraneous bodies, to which it is attached. Tliis causes the 

 shell to off'er many varieties, and the species to be of diflicult 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 

 concavity 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 with that in Pedum, a 

 genus of Pectinidae. 



Fam. 2. Placunidm. This family is represented by the genus Placuna, 

 the shell of which (the only part known) is allied to Anomia and Ostrea. 

 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 Y- 

 shaped tooth, and in the other a corresponding depression ; ligament marginal. 

 This and the preceding family are by some included in the Ostreidae. 



Fam. 3. Ostreidw. The genus Ostrea {pi. Y6, 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 

 t.lie western coast of Africa, where the branches of certain aquatic trees 

 hang in the water at high tide, the oysters afiix themselves to these 



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