398 MOLLUSCA. 



third plate that is sometimes stony and sometimes horny, by which 

 the animal adheres to foreign bodies, and the remainder of it (the 

 muscle) serves to join one valve to the other. The animal, — Echion, 

 Poll, has a small vestige of a foot, similar to that of a Pecten, -which 

 slips between the emargination and the plate that closes it, and per- 

 haps serves to direct water to the mouth which is close to it(l). 



These shells are found attached to various bodies like the Oslrese. 

 They are found in every sea(2). 



Placuna, Brug. 



A small genus allied to the Anomiae, in which the valves are thin, 

 unequal, and frequently irregular, as in the latter, but both entire. 

 Two projecting ribs, en chevron, are seen on the inside of one of 

 them, near the hinge. 



The animal is not known, but it must resemble' that of the Ostrcae, 

 or that of the Anomi3e(3). 



Spondylus, Lin. 



A rough and foliaceous shell as in the Ostrese, and frequently spiny; 

 but the hinge is more complex; besides the cavity for the ligament, 

 analogous to that of the Ostrese, there are two teeth to each valve 

 that enter into fossx in the opposite one; the two middle teeth be- 

 long to the most convex valve, which is usually the left one, and 

 which has a projecting heel, flattened as if sawed .through behind 

 the hinge. The animal, like that of a Pecten, has the borders of its 

 mantle furnished with two rows of tentacula, some of the external 

 ones being terminated by coloured tubercles; before the abdomen is 

 a vestige of a foot formed like a broad radiated disk on a short pe- 

 dicle, and endowed with the faculty of contraction and expansion(4). 

 From its centre hangs a filament, terminated by an oval mass, the 

 use of which is unknown. 



The Spondyli are eaten like oysters. Their shells are frequently 



(1) This foot escaped the notice of M. Poli. 



(2) Jlnomia ephippium, Gm.; — A. cepa,- — A. ekctrica,- — A. squamula,- — Jl. acu- 

 leata; — A. squama,- — A. 2^undata; — A. undulata,- — and the species added by Bru- 

 gieres, Encyc Method., Vers., I, 70, e.t seq. ; and pi. 170, 71. 



The other Jlnomix of Gmeliu are Placunse, Terehratuhe, and Hyalx. 



(3) Jlnomia placenta, Chemn., VIII, Ixxix, 716; — An. sella, lb., 714. See also 

 pi. 173 and 174, Encyc. Method., Vers. 



(4) Called by Poli " the abdominal trachea" in the Spondyli, &c. 



