THE LAMELLIBRANCHIATA. 417 



completely, disunited from one another and from the branchiae, 

 and the latter are either simple plumes or have undergone 

 but little modification. The haemal face of the bod}' is short 

 relatively to its vertical height. 



In most Lamellibranchs the haemal face of the body is 

 longer ; the gills are lamellar, and the mantle-lobes are united 

 with one another and with the gills, so as to separate a supra- 

 branchial from an infra-branchial chamber (Anodonta). In 

 yet others, the posterior margins of the mantle are produced 

 backward into short siphons, but the mantle-lobes remain 

 separate for the rest of their extent ( Cardium) ; in others, 

 the siphons are greatly elongated and the ventral margins of 

 the mantle-lobes unite, so as to leave only a small median 

 aperture for the foot {Pholas). In the most modified forms, 

 the body becomes more and more elongated, until, in Teredo, 

 it is completely vermiform, and the valves of the shell cover 

 but a very small portion of the body. 



The foot is wanting as a distinct structure in Ostrcea / 

 w T hile in Cardium and Trigonia it is a large muscular organ, 

 by the aid of which the animal is able to leap for some dis- 

 tance. The byssus may be present in the young and absent 

 in the adult (e. g., Anodonta). It may have the form of strong 

 chitinous filaments (Mytilus), or of a plate of horny or shelly 

 texture [Area, Anomia). The inequality of the valves at- 

 tains its maximum in the ffippuritidce, in which one valve 

 may have the form of a long cylinder, or cone, while the other 

 is a flattened plate. 1 



The shells of Lamellibranchs are among the most abun- 

 dant of fossil remains in all epochs of the world's history. In 

 the Palaeozoic formations, however, the proportion of these 

 mollusks relatively to the Brachiopoda is the reverse of what 

 obtains at the present day, the latter being very numerous, 

 while the Lamellibranchs are comparatively scanty. The in- 

 tegropalliate are far more numerous than the sinupalliate 

 forms in the older rocks. The Hippuritidw of the Cretaceous 

 epoch is the only family of ancient Lamellibranchs which is 

 extinct at the present day, and the only one which diverges 

 to any considerable degree from existing forms. 



The Odontophoea. — In the Mollusks which belong to 

 this division, the mantle, always present in the newly-hatched 

 young, may abort in the adult condition. It is never divided 



1 For an excellent account of the LamellibrancMata from the conchological 

 side, see Woodward's " Manual of the Mollusca." 



