144 -^ Collide to the Zoological Collections in the 



forming a single structure. This is well seen in the pre- 

 pared specimens of Aspergillum in Case 177C. 



In the Pholads the umbonal reign of the shell is pro- 

 tected by a large single or paired shelly plate, and some- 

 times also the base of the siphons is enclosed in a special 

 shelly cup. The shell in this family, therefore, is always 

 more or less complicated and unlike the ordinary Bivalve 

 shell. Notice the great worm-like tubes of Kuphus in 

 Case 177. 



It should further be noticed respecting the Pholad shell 

 that there are no hinge-teeth, and that the front part of 

 the valves has a rasp-like surface. It is by its own rota- 

 tion, with this rasp-like surface foremost, that the animal 

 bores its way into rock or timber. 



The animals themselves are worm-like : the mantle 

 is closed, except for a small aperture in front for the foot, 

 the siphons are of great length and are usually united 

 throughout their whole extent, and the gills are prolonged 

 into the incurrent siphon. 



ii. LIPOCEPHALA HETEROMYA. 

 [€a0£ 178JI]. 



This Order includes the Mussels, in which the anterior 

 adductor muscle of the shell (see the prepared specimen 

 in Case 172 A) is very much smaller than the posterior, in 

 consequence of the extreme attenuation of the anterior 

 part of the shell. 



In the typical Mussels {Mytilus-A.x).A Modiold) the lobes 

 of the mantle are quite separate, and no true siphons 

 exist; and the foot secretes a " byssus," by which the 

 animal attaches itself to foreign bodies : the shell is long 

 and pointed in front, and is covered on the outside with a 

 thick epidermis, and there are no hinge-teeth. 



Lithodomiis does not merely fix itself to the rocks by 

 its byssus, but actually excavates a narrow burrow in which 

 it lies concealed. 



