PHOLAS. 365 



Tuhicola, differs^ in tlie animals not forming a testaceous 

 tube : and yet, although wanting, some analogy may be 

 traced in certain accessory testaceous pieces, entirely foreign 

 to the shell, and varying in size and number. Some species 

 have also a coriaceous or horny tubular appendage at one 

 end. The family includes two genera — Xz/lophaga and 

 Pholas. 



The Xijlojohaga, generally found in light pieces of wood 

 or stick, which he penetrates to the depth of about an inch, 

 is admirably formed for his subterraneous mode of life. 

 The shell approximates closely in its general structure to 

 that of the last genus of TuUcola. The valves are equal, 

 inequilateral, and very much gaping : they resemble those 

 of Teredo, but in place of the calcareous tube, they are 

 merely furnished with two small accessory calyciform tes- 

 taceous pieces, placed on the anterior side of the hinge. A 

 small curved tooth is also discoverable in each valve, with 

 an internal rib running from the umbo to the basal margin ; 

 two distinct muscular impressions are situated near the 

 edge of the superior margin, the posterior large, the 

 anterior smaller. 



Pholades constitute one of the few genera established 

 by Linnaeus, that remain nearly entire ; the construction 



