LAMELLIBRANCHIATA. 503 



Family Craniadce. 



Shell hinge-less, attached by the umbo or whole breadth of the 

 ventral valve : a short median brachial process. 

 Genera Crania. 



Family Orhiculida. 

 Valves not articulated. Ventral valve perforated for the passage 

 of the pedicle. 



Genera Orbicula {Discina, Lam.), Orbiculoidea, Siphonotreta. 



Family LingulidcB. 

 Shell hinge-less, subequivalve, attached by a longish pedicle 

 passing out between the cardinal end of the valves. Generative 

 sinuses prominent and parallel on the inner surface of the mantle- 

 lobes. 



Genera Lingula, Oholus. 



LECTURE XXL 



LAMELLIBRA^'CHIATA. 



The relation of the contained soft parts to the bivalve shell of the 

 Brachiopoda is such that one valve is ventral, the other is dorsal 

 in position. In the LamelUbranchiata one valve is applied to the 

 right and the other to the left side of the animal. Li the common 

 oyster and the Anomia, which are as fixed and motionless as the 

 Brachiopods, the two lobes of the mantle are as little united with 

 each other, and there is as little evidence of any locomotive organ 

 or foot. The muscles of the shell retain a certain complexity in 

 Anomia, but in Ostrea are reduced to a single adductor.* The 

 spiral brachia are replaced by two shorter and more simple processes, 

 and the inferior labial fold is produced on each side to the same 

 length, so that there is a pair of labial processes on each side of the 

 mouth. These appendages, like the arms in Orbieula and Lingula, 

 have no internal calcareous support, which, by being bent, could 

 open the valves ; but they are long enough, in some species of Ano- 

 mia, to be protruded from the shell. In other Lamellibranchs the 

 labial processes {Jig. 188, d) are short, and are more glandular than 

 muscular. Most of the present acephalous class are free and loco- 

 motive. The instrument by which they move from place to place is 



* Preps. Nos. 65. 

 K K 4 



