744 LAMELLIBRANCHIATA. 



thick and oblong, more or less inequivalve, irregular, and often 

 gaping. The hinge is generally edentulous, and the pallial line is 



interrupted. The genus seems to 

 commence in the Eocene Tertiary, 

 and has continued to the present day. 

 Family 4. Gastroch^nid^. — In 

 this family the mantle -lobes are 

 united ; there are long siphons, 



Fig. 630. — Saxicava rugosa, left valve. • • j - ,i j ,1 c , • r 



Post-Pliocene and Recent. joined together; and the foot is of 



small size, finger-shaped, and not 

 byssiferous. The shell is equivalve, inequilateral, thin, wedge- 

 shaped, and gaping in front. The beaks are placed anteriorly, 

 and the anterior side of the shell is short. The hinge is toothless ; 

 the ligament is external ; and the pallial sinus is deep. The mem- 

 bers of this family burrow in rocks, corals, &c, or bury themselves 

 in mud ; and a longer or shorter shelly tube, to which the shell 

 itself is commonly cemented, is often developed. The tubes and 

 shells of Gastrochana are not very rarely found in the fossil state 

 in rocks of Secondary and Tertiary age, and a number of living 

 species are known. The allied genus Fistularia ranges from the 

 Cretaceous rocks to the present day. 



Order XL Adesmacea. 



In this order the mantle-lobes are united, and there are long 

 siphons joined along almost their entire length. The foot is gener- 

 ally well developed, and there are two adductor muscles. The shell 

 consists essentially of two valves, but accessory plates or an adven- 

 titious calcareous tube may be developed in addition. A portion 

 of the cardinal margin is reflected above the beaks, and the um- 

 bonal cavity is divided internally by a prominent process. The 

 hinge and ligament are not developed. The members of this order 

 are inhabitants of the sea or of brackish waters, and they all form 

 perforations in stone or wood. The order is divided by Fischer 

 into the two closely related families of the Pholadidce and Teredinidce. 



Family i. Pholadid^. — In this family the animal is club-shaped 

 or worm-like, with a short truncated foot, and long siphons united 

 to near their extremities. The shell is gaping at both ends, with a 

 portion of the cardinal margin reflected over the beaks, and usually 

 having the dorsal region protected by one or more accessory plates. 

 The members of this family are almost exclusively marine, and form 

 burrows in stone or wood. In some cases (as in Pholas itself) the 

 young are similar to the adult ; whereas in other cases {Pholadidea^ 

 Teredina, &c.) a marked metamorphosis takes place in development. 

 This metamorphosis consists essentially in the formation of a cal- 



