Mr. W. Clark on the Plioladidse. 829 



open anteally and posteally. The specimen examined measured 

 8 inches from the front valves to the terminal pallets, and 

 when the siphons are extended, an inch longer. The anterior part 

 of the animal is inclosed in a pair of hemispherical shining white 

 valves, with a large angular gape in front, and rounded behind 

 into auricles, which in this species are much larger than in its 

 congeners; the body and mantle are fixed to them, and proceed 

 under the protection of a testaceous tube to the terminal pallets, 

 which are also encased within the tube. 



It will here be convenient to observe, to be spoken of more in 

 detail hereafter, that the calcareous tube through which the body 

 passes, has hitherto been considered as one of mere protection ; 

 but I shall show that though the globular valves in front work 

 free in the tube, it is as much a part of the animal as the shell 

 of the Pholades, inasmuch as it is fixed to it posteriorly by a very 

 strong muscle. 



The branchiae are invisible until the mantle is opened. There 

 is what appears to be a purple dull red labium on each side the 

 mouth, connected by a thin membrane ; these have been termed 

 salivary glands, and may perhaps be such. The oral aperture is 

 subtriangular. The foot in the living animal appears bluish 

 hyaline, but when the moisture is absorbed it is muscular and 

 coriaceous, attached to the body by a thick powerful cylindrical 

 pedicle, and in its centre the terminus of the hyaline stylet is 

 visible ; the form of its basal area is that of the anterior gape, 

 which is of a diamond figure, with its angles placed vertically 

 and transversely, but the transverse axes are longer than the 

 vertical. A pair of yellowish white spatulate appendages are 

 fixed to the posterior extremity of the body. In this animal, 

 besides the anterior and posterior apertures of the shell, there is 

 a rather extensive oval orifice on the dorsal surface of the shell, 

 which is covered by a thick subcircular tough skin, springing 

 from the internal part of the anterior end of the mantle, which 

 appears to have the valvular function of closing the orifice ; but 

 it will be mentioned again. 



These are the only features of the animal which are visible 

 without dissection. A bivalve animal consists of the shell, soft 

 parts, and the hinge, which latter organ has caused some miscon- 

 ceptions, which I will endeavour to remove. It is nearly similar 

 to that of Pholas ; the valves articulate on a thin genuine carti- 

 lage, which is a secretion from glands ; on each side the anterior 

 dorso-lateral part of the body the denticular appliances are want- 

 ing in one valve, and in the other there is only a short blunt 

 tooth ; the ligament is a united production of the glands just 

 mentioned, and the mantle ; it may be considered to be more ex- 

 ternal than internal, and only differs from Pholas in having one, 



Ann. &; Mag. N. Hist. Ser. 2. Vol. vi. 22 



