BIVALVIA. 293 



GASTROCH^ENA FABA. "Leach, MS." 1817. Fide Gray. 

 MYTILUS AMBIGUUS. Dillw. Cat. of Recent Shells, p. 304, No. 9. 

 PHOLAS PUSILLA. Poli. Test. Sic., vol. i, p. 50, t. 8, figs. 12, 13, 1791. 



Spec. Char. Testa elongatd, cuneiformi, tenui, fragili ; hiatu magno ovato; striis 

 Incrementi valde conspicuis ; umbonibus subterminalibus vix prominulis ; cardine subcalloso. 



Shell elongate, wedge-shaped, thin and fragile, with a large ventral gape ; lines of 

 growth conspicuous ; beaks nearly terminal, scarcely prominent ; hinge with an obtuse 

 callosity. 



Length, -f inch. 



Locality. Cor. Crag, Sutton. 



Red Crag, Sutton. Recent, Mediterranean, Britain. 



Numerous fragments of the calcareous tube of this species are met with in the 

 Red as well as in the Coralline Crag ; and I have found the valves imbedded in the 

 globular masses of coral so abundant in the latter Formation, and also in the thickest 

 part of the common oyster. These excavations made by the animal appear all to have 

 been lined with a calcareous coating, as well in the thick shell of the oyster as in the 

 porous substance of the coral ; and the exterior of this marsupium, or purse-like 

 envelope, is always more or less granular, wherever it is exposed.' My specimens 

 present considerable variation in regard to magnitude, some of the valves having 

 twice the length of others, though all appear to have formed a calcareous lining to 

 their excavation, and as such, it is presumed, they have attained to full maturity. I 

 have none so small as to be considered the young, without the lining, assuming it not 

 to have the power of increase after it has once formed its own envelope.* 



This calcareous flask-like covering is terminated posteriorly with a tube for the 

 protection of the siphons, which project a little beyond the surface of the coral, and 

 the neck of this is often of considerable thickness. At a distance of about a quarter, 

 or sometimes half an inch within the tube, a calcareous partition is seen (fig. 1 1 d), 

 with a linear opening crossed in two places, assuming the form of a double dagger, 

 each siphonal aperture having the appearance of a cross. This opening appears to be 

 variable in different species, but whether the form be a good specific distinction I am 

 not able to say. 



* In the case of the Gastrocfuena, it is probable that only when it has attained to full maturity does 

 it form a lining to its crypt, or a covering to itself ; this being, as it were, exterior and detached from the 

 vital portions of the animal, may be incapable of alteration, but it probably possesses the power of dissolving 

 or destroying this case, and constructing another, as occasions require, by the same means it has employed 

 for the enlargement of its domicile. The prevailing opinion is, that the shell, or exo-skeleton, as it is called, 

 being extra-vascular, is not susceptible of alteration by interstitial increase, but we know that absorption 

 does take place ; and the examination of shell-structure by the microscopist shows us clearly its highly 

 organized condition, permeated with vessels for the conveyance of fluids ; and it is exceedingly difficult to 

 explain in any other way some of the operations of the Mollusca, more especially the alterations in magnitude 

 of the hinges of the Bivalves, for example, during the growth of the animal. 



