161 



Figure. 



Glycimeris siliqua. PI. 43. Fig. 234. Shell, showing its widely 

 gaping extremity and shining-black horny epidermis. 



Genus 13. PHOLADOMYA, Soioerby. 



Animal ; with the mantle thin, closed except in front for the pas- 

 sage of a small foot, which has an accessory bifurcate foot ; 

 siphons long and large, enclosed within a coarsely wrinkled epi- 

 dermic sheath, the branchial one having at its base a supple- 

 mentary valvular aperture ; branchiae undivided. (Owen.) 



Shell ; oblong, thin, ventricose, pearly within, very inequilateral, 

 anterior end very short, rounded ; posterior elongately fan- 

 shaped, gaping ; hinge toothless, composed of a slight fulcrum 

 supporting a moderate external ligament. 



In Pholadomya we have another instance of a long-siphoned, close- 

 mantled, mud-inhabiting bivalve, possessing in like manner a shell with 

 an external ligament supported on a ledge or fulcrum. But notwithstand- 

 ing the similarity between the organization of the soft parts, as well as the 

 shell's hinge, of Panopaa and Glycimeris, and those of Pholadomya, the 

 difference in the form and substance and general aspect of the shells is 

 very great. The shell of Panopaa is comparatively solid, almost colour- 

 less, and destitute of any sculpture beyond a multitude of somewhat 

 irregular lines of growth ; the shell of Glycimeris is of a peculiar opake 

 chalk-white substance, very dense, secretions from thicker mantle-lobes, 

 and covered in a conspicuous degree externally by a shining-black horny 

 epidermis. The shell of Pholadomya is a light, ventricose, papyraceous 

 shell, symmetrically cancellately ribbed, covered with a light brown epi- 

 dermis externally, and internally slightly nacreous. 



Genera of Mollusca are often unavoidably founded in error. The shell 

 is generally the first portion of the creature brought to light, and the con- 

 chologist has no alternative but to guess at its affinities by analogy. Mr. 

 Sowerby considered the shell of Pholadomya to partake of the characters 

 of the genera of which he compounded the name. When a specimen col- 

 lected alive at the island of Tortola by Mr. Stutchbury was submitted to 

 Professor Owen for dissection, it was comparatively easy to answer " the 

 question requiring a reply " of M. de Blainville, " Qu'est-ce que le genre 

 Pholadomie de quelques auteurs Anglais ?" The animal was found to have 

 the nearest affinity with that of Panopcea, distinguished nevertheless by an 

 accessory bifurcate foot, by the presence of a supplementary valvular aper- 



VOL. II. Y 



