PHOLAS. 



triided into it; for although Lamarck has supposed the 

 family might contain several distinct genera, he has not 

 attempted any division ; Leach, it is true, undertook to 

 divide the Linnean Pholades into several genera, but as 

 his distinctions consisted principally in the number of 

 what have usually been called the accessory valves^ and as 

 almost every species differs in the number of these, we 

 have not thought it necessary to adopt any of his genera: 

 they appear calculated for divisions of the Genus, but 

 not sufficiently strong as generic distinctions. There are 

 some species, the Pholas clavata Lam, for instance, 

 which, on account of their being closed at both ends, it 

 might be proper to distinguish as a Genus, because this 

 circumstance implies a difference in the habits of the 

 animals by which they are formed ; this character has 

 therefore been seized by Leach, and upon it he has con- 

 stituted his Genus Martesia, and we should have been in- 

 duced to have followed his example in this instance, had 

 we been convinced of its necessity, 



The Pholades may be described as transversely ob- 

 long, equivalve, very inequilateral bivalves, vt^hose exter- 

 nal surface is generally rough, like the surface of a rasp 

 or file, with muricated striae or ribs v/hich diverge from 

 the umbo and pass to the margin ; when the valves are 

 closed, they gape at both ends (in most species) but the 

 opening is very large at the anterior end in the greater 

 number, and extends along the margin; but in some spe- 

 cies it is almost closed by an addition of testaceous mat- 

 ter to the edge of the shell, over which the radiating 

 murjcr.ted striae are not continued. We believe all the 

 shells of this Genus are furnished with a greater or less 

 number of accessory valves, which appear to be caused 

 by the deposition of shelly matter (within the epidermis 

 and connected with the valves by that membrane) wher- 

 ever such valves were necessary for the security of the 

 inmate ; they are consequently very various in form, and 

 placed in different situations in the different species, 

 though in most cases they are placed near the hinge, and 

 have even been considered to be substitutes (in these 

 shells) for the permanent ligament of other bivalves : we 

 must for the present withhold our assent from this opi- 

 nion, because, on account of the situation in which they 

 live, the animals inhabiting these shells can have very 



