880 Mr. W. Clark on the Pholadidse. 



the upper, instead of two layers of transverse fibres, strengthened 

 and covered as in that genus by the anterior end of the mantle 

 being reflected on it, but it is not fortified by testaceous plates. 

 We have here all but the hinge of Pholas, and taking the shell as 

 far as its circumscribed volume extends, we find it nearly similar 

 in having the curved subumbonal internal apophyses, the single 

 post-medial adductor, and the long tubular mantle fixed to the 

 auricles ; but instead of the viscera and branchiae being inclosed 

 in the usual bivalve portion, they are placed in that part of the 

 mantle which is external to the shell ; nevertheless they are pro- 

 tected by the tubular case, which, as I have stated, is an integral 

 portion of the hard parts of the animal, not merely protective or 

 accessorial. We have thus a complete equivalent for the bivalve 

 shells of the Pholades, in which the siphonal apparatus commences 

 at the posterior end of the shell, deriving their retractors from 

 offsprings of the medial adductor ; whereas in Teredo the retrac- 

 tors have their source from a particular muscular sphincter at the 

 posterior end of the tubular mantle in which the pallets are in- 

 serted, and have nothing of the nature of an adductor muscle, as 

 the tube to which they are fixed is a perfect cylinder. 



The next point to engage attention is the muscular structure, 

 which, with slight exceptions, scarcely diff'ers from Pholas. The 

 two principal masses of muscles are those of the foot and the ad- 

 ductor ; the latter is a powerful fibrous mass of bright red fila- 

 ments, as Sir Everard Home states was the colour of the species 

 he examined; it embraces the hinder part of the mantle within 

 the hemispherical valves, being post-medial and fixed in the in- 

 ternal hollows of the auricles, showing therein when removed 

 well-marked cicatrices : this muscle throws off elastic ribbons, 

 which proceed on the lateral parts of the mantle to that point of 

 the tubular mantle where the sphincteroidal muscle is fixed, and 

 of which it is probably the origin : this last muscle is a most im- 

 portant one ; by being permanently fixed to the animal and pos- 

 terior end of the protective tube, by the oval-shaped fillets spring- 

 ing from the sphincter muscle, it is the point of support for the 

 retractors of the comparatively short siphons, and also the fulcrum 

 for the pallets that are firmly fixed laterally therein, and undoubt- 

 edly serve to compress and relax the siphons. It is necessary to 

 observe, that if the very long mantellar tube was not firmly at- 

 tached, the points d'appui of the pallets and retractors would be 

 lost, and the long, linear branchiae drawn together in the tube in 

 such a mass as to impede the passage of the water and other 

 functions. The posterior sphincter in Dentalium is analogous in 

 its uses; and though the hemispherical valves of Teredo play 

 loose in the anterior part of the tube, they are kept in proper 

 position by the powerful operation and suction of the foot, and 



