P H L A S. 



Genus PUOLA.s. UH,Heus. 

 Testd mollusci acephali terehranlis hivalvis, liimusctilosd, 

 plenivique suhcylindraced, postiri atteiiualA, valvis 

 wntice hiantibiis, vel integumenti clausis, margine 

 (loisali laminis testaceis acceMoribits und vel 2)biri- 

 mis tectis ; intks infra umbones utrdque valod 

 jrrocessu arcuato siib wnbonc ajjixf). 

 yhell of an acephalous boring mollusc bivalve, binius- 

 cular, generally subcylindrical, attenuated poste- 

 riorly : valves gaping in front or closed with an 

 integument; dorsal margin covered by one or 

 more accessory shelly plates; an arched process 

 fixed to each valve within under the umboes. 

 The Fholadce have been usually divided accord- 

 ing to the number of accessory plates, or the shape and 

 disposition of the terminal integuments. Whatever may 

 be the merits of such division, in this monograph the 

 various forms will be united under one generic term. 

 It is curious, however, to observe how the species pass 

 from the simplest form as represented by such species 

 as Pholas parva, in which the hiatus in the valves is left 

 open with a single dorsal plate, tlirough those which have 

 a complicated arrangement of plates, to those which, 

 like Pholas calva, are closed at one end by a shelly 

 covering over the whole anterior, and at the other end 

 by marginal integuments. Then come those which, 

 like Pholadidea papyracea, have a cup or pair of flaps 

 at the end of these integuments, followed by those 

 which, likePAo/rts tubifera and triden!t, have a shelly tube 

 at the end of the cup. The prevailing opinion respect- 

 ing the manner of boring among the Fho/ada:, is that 

 the sucker foot holding to the stone, wood, or wax to 

 be bored, and forming a pivot on which the shells 

 rotate, the STibstance is bored by the rasping power of 

 the sharp points or prickles of the shell. I should ascribe 

 only a part of the work to this process, as assisting other 

 agencies. Other bivalve mollusca bore which have no 



such rasping prickles on their shells. The Pholadu: 

 are found in most parts of the world, and the species 

 are very widely spread, especially those which, like 

 Pholas striata, bore in wood, and being transported by 

 ships and other floating timber become cosmopolitan. 



.Species 1. (Mus. Jeffreys, &c. ) 

 I'uoi.As c.\Nr>iD.\. Pko. testd subventricosd, ienui, (ui- 

 lice rutundatd, medio ventricosd, costis acute imbii- 

 catis radiatd, postice attenuatd, acuminatd, lavi- 

 gatd ; lamind dorsali uiid, aiiyustd, siinjilici. 

 Tin: vvjiiTK Piiouvs. Shell subventricose, thin, rounded 

 in front, ventricose in the middle, rayed with 

 sharply imbricated ribs, posteriorly attenuated 

 acuminated, narrow, simple. 

 LlNNjEUs. Systema Natura;. 

 Ilab. Great Brit^-jin. Burrowing in chalk. 



Differing from P/colas parva and others of the single- 

 plated group in the roundness of the anterior termi- 

 nation of the valves. 



Species ■>. (Mus. Brit.) 

 PliOLAS COSTATA. Plio. (csld mayii(},iii/,,,iii.<l<inyatd, 

 ventricosd : costis i-alidis suhrotiiiiilniis imhricatis 

 ovtnino radiatd; laiiiind dursnli tnijiuj^dari, 



The ribbed Piiulas. Shell largi , open, elongated, 

 ventricose, rayed with strong, rather rounded 

 imbricated ribs ; dorsal lamina triangular, horny. 

 LiNN^us. Systema iS'aturffi. 

 Hah. Mexico and United States. 



This very handsome species has no shelly dorsal 

 plate, but a thin horny triangular integument spread 

 on a platform, formed by the reflected margins of the 

 valves. 



Angus 



