MONOGRAPH OF THE GENUS 

 PHOLAS. 



Char. Gen. MoUusciun aceplialmn terebrans. Testa bival\ds, ple- 

 rumque subcylindracea^ tenuis, alba, plerumque scabra, epidermide 

 induta, valvse sequales, infra umbones processis duobus arcuatis. 



A well-known genus of boring Mollusca, with an sequivalve, 

 colourless shell, of a more or less cylindrical form, generally 

 covered with an epidermis, which, in some species, is so thin as to 

 be scarcely perceptible. Most of the species have a kind of protecting 

 shield on the umbones, or between the dorsal edges, consisting of 

 one or more laminated pieces. Some species are gaping at the 

 anterior ventral margin ; and in others, when mature, the liiatus is 

 closed by a more or less horny lamina. In the species separated 

 by some authors under the generic name Pholadidcea, the posterior 

 integuments closing the valves are lengthened at the termination 

 and formed into a cup, and in others this cup surmounts a shelly 

 tube, formed for the protection of the posterior tubes of the 

 animal. In one character all species of true PJiolades agree, and 

 that is, in having a pair of curved processes advancing from under 

 the umbones. In the genera Xylophaga and Triomphalia these 

 are wanting. 



1. Pholas Dactylus (pi. cii. f. 10, 11, pi. cv. f. 47), Linn. 

 1110. C/^m;^. f. 857. 



Ph. testa aperta, elongata, postice subattenuata ; costis angulatis, 

 acute imbricatis, versus extremitatem evanidis ; laminis dorsalibus 

 duo magnis anticis, una elongata postica. 



Elongated, with the anterior ventral margin widely gaping, and 

 ending in a point ; ribs angular, sharply imbricated, spreading 

 over the anterior and middle parts of the shell. The two broad 

 accessory valves on the umbones have the nucleus posterior and 

 lateral; one long posterior valve. Pig. 47 represents a mere 

 dwarf distortion. 



In sandstone, at low water. Many parts of Great Britain. 



3 ft 



