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only very rudimentary teeth or ligament, capped externally with 

 an accessory plate or plates. 



The species of this group have very strongly marked characters, and 

 may be distributed either into a dozen genera, or into one only. For the 

 present we adopt the latter course, uniting all the varieties of forms, in- 

 cluding even Xglophaga, under P kolas. The hinge of the shell has only 

 a very rudimentary indication of teeth or ligament, and is, with rare ex- 

 ception, capped externally by one or more accessory shelly plates. An- 

 teriorly the shell more or less widely gapes for the passage of a largely 

 developed foot, which aids in the boring habits of the creature ; posteriorly 

 it gapes a little for the outlet of the siphons, which are extended in an 

 epidermic sheath. 



Genus 1. PHOLAS, Linnceus. 



Animal ; thick, club-shaped, with the mantle united except in front, 

 for the passage of a conspicuously developed sucker-like foot, 

 which is reflected dorsally over the umboes to form callosities 

 and accessory plates ; siphons long, united within a fleshy 

 sheath, divided at the extremities ; orifices bordered with cirrhi, 

 sometimes surrounded by a cirrhated disk. 



Shell ; ehiigately auger-shaped, equivalve, gaping especially in 

 front ; apertures sometimes closed at maturity by a shelly wall, 

 outer surface scidptured anteriorly with ridges in prickly fes- 

 toons, posteriorly smooth ; hinge composed of a rudimentary 

 superficial ligament with external accessory plates of different 

 forms in connection with it ; umboes mostly covered externally 

 by a reflected callosity, internally furnished with a curved hook- 

 like apophysis. 



The Pholads, or Borers, are a tribe, of the true nature of which mere 

 collectors of shells are liable to form a very erroneous impression. The 

 habit of the mollusk involves an entirely new construction of hinge; the 

 valves of the shell are not connected by interlocking teeth or by elastic 

 ligament, but by a prominent hook, called an apophysis, springing from 

 within the umbo of each valve, and entering into the flesh of the animal 

 in connection with the pedal muscle. Teeth and ligament are quite rudi- 

 mentary ; outwardly the hinge is capped with variously shaped accessory 

 plates, and either the plates may be wanting in the collector's specimen, 

 or, what is more commonly the case, the plates may be glued to the spe- 



