Mr. W. Clark on the rholadid«. 317 



obscure and rudimental, and M. Deshayes, in his comment on 

 Pholas in the last edition of Lamarck, mentions the hinge as 

 scarcely existing, and not being a ' veritable ligament ' — how dif- 

 ferent from the fact ! And I will observe, that if there is a genus 

 better provided than any other of the bivalves with ligamental 

 appendages, it is Pholas. 



The hinge of Pholas dadylus has very slight traces of denti- 

 cular assistance ; it nevertheless works, en charniere, in a circum- 

 scribed space, to which it is confined by powerful ligaments, and 

 though somewhat different in its component parts from the usual 

 configuration, it does not in its functions materially differ from 

 those of the ordinary bivalves ; it has a strictly internal cartilage, 

 which is laminar, of small volume, oval shape, and light yellow 

 coloui' ; it is fixed on the internal portion of the convexity of the 

 valves, termed the hinge, which articulates, imbedded in the thin 

 plates of the cartilage. The ligament succeeds ; it consists of two 

 parallel plates, between which is a considerable interspace of 

 strong, close-set, white elastic transverse threads, the one fixed 

 more externally to the inner side of the reflected dorsal cellular 

 excrescence ; the other, below it, to the internal commissure of 

 the two valves, thus forming a powerful ligament that allows 

 them the usual movement of the ordinary hinge : on this is added 

 a third ligamental apparatus, which may be termed accessorial, 

 to increase the strength of the hinge, and is formed by the re- 

 flection of the tough end of the mantle issuing between the ante- 

 rior points of the valves in an elongated oval form, and covers the 

 transverse threads of the outer layer of the ligament ; it is firmly 

 secured by throwing out filaments which enter the dorsal cells of 

 each valve ; this production of the mantle is further fortified by 

 two thin, flexible, suboval testaceous plates, supported by a sub- 

 triangular rest ; these appendages are exudations from the re- 

 flexed mantle. The posterior part of the valves, as is usual in 

 elongated shells, has the common continuous membranous liga- 

 ment produced by the protrusion of the edge of the mantle, with 

 the addition, in this species, of a long thin linear testaceous 

 plate ; the use of this posterior ligament is to assist in maintain- 

 ing the valves from sliding out of their natural position. It ap- 

 pears then that Pholas is iron-bound as to ligament, which, 

 in it, is far more powerful in securing the valves than in the 

 shells of any other group of the Acephala of similar fragility and 

 tenuity. 



The Mtiscular System. 



It will now be convenient to notice the muscular system, and 

 in the first place, that part of it connected with the shell. In 

 this group of bivalves, the curved spatulate apophyses springing 



