Lithophaga.] PELECYPODA. 871 



Colour uniformly dark chestnut. Interior bluish-white, purplish at the 

 posterior end, faintly shining, and very slightly pearly. Margins 

 smooth, sharp, the epidermis bent over them. Hinge linear, smooth. 

 Ligament about one-third of the length of the valves, internal, but 

 little of it visible from the outside. Anterior adductor-scar elongate, 

 the posterior scar round and united above with the narrow and short 

 byssus retractor scar. Pallial line simple. 



Diameter Ant. -post., 36 mm. ; dorso-ventral, 15 mm. : thickness, 

 16 mm. 



Type in the British Museum. 



Hob. North and South Islands. Boring in hard rocks between 

 tide-marks ; sometimes it is boring into the massive shells of Glycymeris 

 laticostata. 



Genus 5. DACRYDIUM, Torell, 1859. 



Dacfijdiinn, Torell, "(Spitsbergen's Molluskenfauna," 1859, 138. Type : 

 Modiola (?) vitrea, Moller. 



Animal with the ventral side open ; foot byssiferous. 



Shell oval, trapezoidal, very short anteriorly, dilated posteriorly ; 

 surface generally smooth ; beaks contiguous ; margins simple ; liga- 

 ment internal in a triangular resilifer under the beaks ; hinge fairly 

 strong, deeply plicated on both sides, convex below, interrupted under 

 the beaks ; impression of the anterior adductor lanceolate and marginal, 

 of the posterior adductor suboval. 



Distribution. Arctic and Antarctic seas ; Atlantic ; Australasia : 

 in depths to about 2,500 fathoms. 



Remarks. The animal of Dacrydium lives in an elongated tubular 

 nest woven of byssus threads and covered by Foraminifera, spicules of 

 sponges, &c. 



KEY TO SPECIES. 



A. Shell smooth . . . . . . . . . . . . Pelseneeri. 



B. Shell radially finely ribbed . . . . . . . . . . radians. 



1. Dacrydium Pelseneeri, Hedley, 1906. Plate 51, fig. 18. 



Dacrydium Pelseneeri, Hedley, T.N.Z.I., xxxviii, 1905 (1906), 72, pi. 2, f. 8. 



Shell small, thin, translucid, with a nacreous lustre, oblong inflated, 

 straight on the anterior side, rounded dorsally and ventrally, almost 

 angled at the anterior dorsal corner. Umbo slightly projecting. 

 Sculpture regular-spaced elevated growth-lines. A thin membranous 

 epidermis clothes the valve. Hinge with a few anterior teeth, and a 

 long row of posterior teeth, which increase in size as they recede from 

 the chondrophore. 



Height, 2-2 mm. ; length, 1-48 mm. 



Type in the Dominion Museum, Wellington. 



Hob. Off Great Barrier Island, in 110 fathoms ; a pair of valves. 



Remarks. The species appears to differ from D. albidum, Pelseneer, 

 by its rough surface and by its greater length in proportion to height. 



