PHOLAS. 



» 



little occasion to open their valves. A long, eurveiJ, 

 sometimes hooked, rather flat, sometimes expanded and 

 somewhat spoonshaped tooth is found in each valve in- 

 serted beneath the hinge margin, and as it were growing 

 out from the inner part of the umbo : this tooth is pecu^ 

 liar to Teredo Siud Pholas.. The anterior dorsal margin of 

 the shell in the region of the umbon^s, is reflected, as we 

 believe, in all the species ; in some cases it is simple, and 

 laid down almost close upon the umbones, in others a 

 second margin is produced, which is placed at a distance 

 from the first, and the space between is divided by a 

 greater or smaller number of entire transverse septa. 

 Whether or not there is any permanent ligament in this 

 Genus, as we have never obsei'ved the animal alive, we 

 cannot undertake to determine ; Turton says it has none ; 

 Lamarck, on the contrary, speaks of the accessory valves 

 covering and hiding the ligament. As far as we can form 

 an opinion from dried specimens, we cannot consider the 

 substance to which these valves are attached as the liga- 

 ment, but as part of the adductor muscle, nevertheless 

 we think we can in some species perceive a very small 

 internal ligament, attached to two unequally sized small 

 curved teeth (one in each valve) placed in the same situa^ 

 tion as the hinge teeth of common bivalves. The ad- 

 ductor muscle forms two principal impressions, one of 

 which is placed on the reflected margin, over the um- 

 bones, and the other about half way between the umbones 

 and the longer end of the shell; there is also a large 

 sinus in that narrower part of its impression by which the 

 mantle is aflixed, and at the angle that is foraied by this 

 sinus, very near the basal margin of the shell, the im- 

 pression is somewhat expanded. 



The pi'incipal differences between Pholas and Teredo 

 consist in the latter forming a shelly tube behind its 

 valves, and in its being destitute of accessory valves>, 

 moreover the two valves of this latter^ when closed, are 

 nearly globular : the same characters distinguish Pholas 

 from Xylotrya of Leach: Xylophaga of Turton, which 

 has accessory valves, and which does not form a shelly 

 tube, is how^ever destitute of the internal curved tooth, 

 which is common to Pholas and several Tubicolees, 



There are many recent species of Pholas, some of 

 which are very abundant in the rocks of our coasts 3 of 



