REPORT ON THE LAMELLIBRANCHIATA. 253 



This species must not Ije conlbuiuled with Pectitnmlus laticofitatus, Quoy and 

 Gaimard, a form common in some parts of New Zealand. That species is Ijroader at 

 the vipper part, and has many more and finer ribs, lacks the depression down the front 

 of the valves, has fewer hinge-teeth, more marginal denticles, and im Inown mark on 

 the hinder muscular scar, besides other minor diti'crciices. 



It is possible this may be the Pectunculus ^fiabdlatiis of Tenison-Woods (Trans. 

 Roy. Soc. Victoria, 1878, vol. xiv. p. Gl), but I cannot (piite reconcile it with the 

 description. I do not find the number of ribs ever amounting to tiiirty-five, nor do 

 they "become very close at the sides as the shell grows." If by "alt." Mr. 

 Tenison-Woods means the diameter of the closed valves, I sliould think he has given 

 the wTong measurement (44 mm.) under this head, or else his character " tinnidiuscula" 

 must be incorrect. Pectunculm heddomei is only al)out lialf that extent in diameter, 

 and decidedly flatter than Pectunculus htticosfatn.';. 



No mention is made of the peculiar dark In-owu muscular scars in Pcctumnlus 

 flcihellahis, which is a prominent feature in the present species, the interior lieing 

 described as if entirely of an " intense fulvous livdwn." 



Pectunculus lutirus, Lamarck. 



Pectuncnhts litreiw, Lamarck, Aiiim. sans vert., ed. 2, vol. vi. j). 49.5. 

 Pectunculus vitreus, Reeve, Condi. Icon., vol. i. pi. viii. Kgs. 45, a, 1j. 

 Pectunculus vitreus, Ilanley, Cat. Eec. Biv. Shells, p. 165, pi. xix. tig. 2S. 



Habitat. — Station 188, soutli of New Guinea, in 28 fatJioms (Challenger) ] Cape 

 York (Brit. Mus.). 



The ribs of this remarkal)ly flattened species are two or three times more numerous 

 towards the outer margin than near the umbones, which results from their being 

 dichotomous when the shell is young, and other intervening ones being produced during 

 growth. 



When in fresh (jondit:ion the radiating slender costie are flnely nodose, and the 

 sulci between them are very distinctly concentrically striated. The interior of the 

 valves is either entirely white or more or less stained with l)rii\vn. and is also A*eiy finely 

 radiately ridged. 



The hinge is very peculiar, consisting of two all but straight series of teeth, which 

 convero-e to an angle of about 105°. The teeth arc about fourteen on cai-li side, mostly 

 o-rooved at the top, and placed almost parallel with the dorsal margin. The .sculpture 

 and hinge of Pectunculus norce-guineensis, Angas, is precisely of the same character as 

 in this species, and I am of opinion that when our series of specimens is sutticiently 

 large, it will be found to be onl}- a variety, dift'ering from the XjV^ merel}- in outline. 



tD 



