Nvcula.] PELECYPODA. 831 



Fam. NUCULID^, Gray. 



Animal with free mantle-lobes, without siphons ; pallial line simple ; 

 the labial palps free, very broad, and provided with a posterior ap- 

 pendage ; all the branchial filaments are oriented transversely. 



Shell compact, closed, with the teeth in two series meeting below 

 the umbones, separated by a chondrophore ; area represented by an 

 obscure lunule and escutcheon ; no ligament, but a wholly internal 

 amphidetic alivincular rcsilium ; internal layer of shell nacreous. 



Ordovician to Recent. 



Genus 1. NUCULA, Lamarck, 1799. 



Nucula, Lamarck, Mem. Soc. Hist. Nat., Paris, 1799, 87. Type: Area 

 nucleus, L. Nuculana, Link, 1807. Glycymeris, Da Costa, 1778 (in part). 



Animal with the mantle open throughout its length ; labial palps 

 very large, subtrigonal ; heart situated on the dorsal side of the 

 rectum ; foot very large, the sole, when extended, forming a disc 

 with denticulate margin ; branchiae very small, narrow, unequal ; 

 no syphons and no byssus. 



Shell oval or elongate, equivalve, inequilateral, posterior side very 

 short ; umbones inclined posteriorly (opisthogyrate) ; surface smooth 

 or striated ; hinge angulate ; ligamental pit internal, triangular, 

 and on each side are developed numerous compressed sharp teeth ; 

 interior of the valves nacreous, adductor impressions subequal ; 

 pallial line simple. 



Distribution. In all marine zones, living in inud, sand, and gravel ; 

 over 70 species, some of them abysmal. 



Fossil. Palaeozoic to Tertiary. 



Vernacular Name. Nut-shell. 



KEY TO SPECIES. 



A. Valves concentrically sulcate . . . . . . . . Bartvigiana. 



B. Valves smooth, shining. 



a. Margins smooth ; valves considerably longer than high . . Strangei. 

 aa. Margins crenulated. 



b. Posterior end angular ; lunule and escutcheon indis- 

 tinct ; length and height equal . . . . nitidula. 

 bb. Posterior end broadly rounded ; lunule and escutcheon 

 distinct, bounded by low ridges ; somewhat longer 

 than high . . . . . . . . castanea. 



1. Nucula castanea, A. Adams, 1856. Plate 51, fig. 1. 



Nucula caslanea, A. Ad., P.Z.S., 1856, 53 ; Conch. Icon., xviii, f. 19 ; M.N.Z.M., 

 164. 



Shell small, oblique, subtriangularly oval, equivalve, inequilateral, 

 almost smooth, shining, subcompressed, chestnut. Beaks small, 

 rounded, approximate, directed backward, situate at about the 

 posterior third. Anterior end sloping, lightly curved, rounded on 



