192 MOLLUSC A. 



Mr. Owen. In Orbicula two small ganglia were found on 

 the side of the oesophagus next the perforated valve ; from 

 which two filaments, accompanying the oesophagus through 

 the membranous wall, immediately diverge and pass ex- 

 terior to the anterior shell muscles, proceeding with corres- 

 ponding arteries to near the heart, beyond which he could 

 not trace them. A single small ganglion is situated on the 

 opposite side of the oesophagus, but on a plane posterior to 

 the preceding ; this is probably the cerebral ganglion for 

 giving off nerves to the free spiral extremities of the arms, 

 close to the base of which it is situated. 



ORDER II BIVALVIA. 



Mouth destitute of fringed spiral arms. 



The animals of this group form the class Conchifera of 

 Lamark, the Bivalvia of the older naturalists. 



The shells exhibit great variety of form and relative size. 

 They are joined together at the hinge, which is either plain 

 or toothed, and corresponds in position with the back of the 

 animal. The connection of the two valves is secured by 

 the intervention of an elastic horny ligament, the office of 

 which is to keep the valves open. It is either external or 

 internal. The valves are closed by means of adductor 

 muscles, intermixed with tendons, and, passing traversely 

 through the animal, adhere to the corresponding places in 

 the inside of each shell. By the contractions of these 

 muscles the free edges of the valves are brought into con- 

 tact, at the same time that the ligament is compressed or 

 stretched, according as it is internal or external. The num- 

 ber of muscular impressions is employed by Lamark in the 

 division of the Bivalvia into two orders, Dimyaires and 



