THE MOLLUSCA OF TORRES STRAITS. 203 



smoothisl), oblong, and slightly gaping anteriorly, tho posterior 

 portion of tlie valves closed in repose. The umbones are hardly 

 prominent, and incline forwards, being closely approximate ; the 

 anterior margin is at first sloping, then somewhat trancately 

 rounded off, and merging into the ventral margin, while 

 posteriorly it is slightly produced and gently rounded. The 

 surface of both valves is alike, smoothish, milky- or ashy-white, 

 semipellucid, indistinctly but rudely concentrically striate, and 

 closely beset with radiating quasi-internal riblets, hardly standing 

 out beyond the superficies ; indeed in young examples these 

 radiations appear as pellucid lines imbedded in the internal 

 layers of the sbell. The ligament is external. Within, the 

 binge of either valve is almost destitute of teeth or any processes 

 whatsoever, save tbat a small, worn-looking, centrally situated 

 lamellar tooth is present, most conspicuous in the left valve, 

 though there is a corresponding tooth and an elongately 

 triangular pit in the right valve also. 



A few remarks on this peculiarly interesting genus may not 

 be out of place here. 



Pholadomya, instituted by the elder Sowerby in 1823, is one 

 in which but very few recent species are embraced, since certain 

 forms described by Agassiz as nearly akin to the type (for example, 

 P. caspica) have been, and rightly, relegated to the toothless 

 section of the Cardiidae, e. g. Adacna, Eichwald. 



P. Candida, Sow., the original type, is still of extremely in- 

 frequent occurrence, nearly all the specimens hitherto obtained 

 having been cast ashore from considerable depths after storms 

 and hurricanes off the island of Tortola, in the Antilles. The 

 anatomy has been worked out by Sir Eichard Owen, who de- 

 scribes the mantle-margins as being united, save where space is 

 left for the foot and outlets occur for the siphonal and anal 

 orifices ; the gills on either side single, the outer lamina produced 

 dorsally, and there is also an accessory, bifurcate foot. A fine 

 example of the shell in the private collection of one of us 

 exhibits a large, papyraceous, fragile, white, extremely insequi- 

 lateral surface, equivalve, very convex centrally and towards the 

 umbones, widely gaping both anteriorly and posteriorly. In 

 form obliquely trapezoid, anteriorly roundly truncate, concen- 

 trically rudely striate, and, centrally only, rayed from the 



LINN. JOUEN. — ZOOLO&T, VOL. XXVII. 15 



