Dr. J. E. Gray on some Families of Bivalve Shells. 27 



cular central scar, formed by the ligaments of the muscles which 

 suspend the mantle and gills. 



3. Vulsellidai. Anterior adductor muscle none. Gills narrow, 

 much produced behind, united together and to the inner surface 

 of the mantle, dividing the mantle-cavity into two parts. Rec- 

 tum simple. Vulsella. 



In all these families the body forms a single mass, the tube 

 of the rectum passes over the back of the adductor muscle, the 

 vent being free and medial. 



Ostreina. Add, vent medial, free ; the body forms a single cen- 

 tral mass. 



Ostreidce. Gills united together and to the inner edge of the 

 mantle. Shell, hinge toothless. 



Plicatulidce . Gills free behind and free from the mantle, sus- 

 pended from the body by a membrane. Shell, hinge with two 

 diverging cross-grooved cardinal teeth. 



The genus Plicatula, which has been hitherto placed with 

 Spondylus, should be removed to the tribe Ostreina and formed 

 into a separate family, as the animal has no appearance of any 

 foot, which is so peculiar in the former genus. The animal is 

 very like Ostrea, has four equal suspended gills united together, 

 acute, and produced beyond the lower side of the adductor 

 muscle. Lips four, rather small, united together above the 

 rather large mouth. The shell is attached by the outer sur- 

 face of the left valve, and the hinge is furnished with two diver- 

 ging teeth, with the cartilages in a triangular pit between their 

 base. 



Anomiaina. Foot distinct, small, truncated at the end ; ovaries 

 separated from the mass of the body and attached to the inner sur- 

 face jaf the right leaf of the mantle. Vent nearer to or attached to 

 the right leaf of the mantle. Gills united together behind, suspended 

 by membranes to the inner side of the mantle. 



Anomiadce. Animal attached, rather distorted; foot on the 

 right side of the body, with a very large byssal pore at the base ; 

 byssus horny or stony, formed of parallel laminse, emitted 

 through a notch in the right valve of the shell. Pedal muscles 

 large, leaving two or three large scars on the left valve. The 

 byssus, or plug, is placed in exactly the same situation in the 

 animal as the beard or byssus of Mytilus, Pinna, &c., and the 

 animal is only rather distorted by being more closely attached to 

 the marine body than in those genera. 



It shows that what is called the foot of the Arcs is in fact an 

 enlargement and production of this byssus-forming organ, while 

 the real foot is greatly reduced. 



