58 AXEL A. OLSSON 



Family NUCULANII>AE (Ledidae) 



Shell small or medium-sized, mostly porcellaneous, elongate, the pos- 

 terior side longer, narrower and often rostrate at the end, the anterior side 

 shorter, rounder and convex. Hinge and ligament as in the Nuculidae, the 

 chondrophoral pit usually internal but sometimes migrating upwards to the 

 dorsal margin to become partly external. Pallial line more or less sinuous. 



Two genera may be recognized. 



I. Shell with the posterior side much narrowed, often arcuate and pointed 

 at the end, the anterior side higher, rounded at the end and often 

 convex. Posterior-dorsal submargin, subelliptical in shape, flattened or 

 concave (rostrum). 



Genus Nucidana 

 II. Shell eloneate, lanceolate, depressed, the posterior side not strongly 

 rostrate. Rostrum absent or very narrow. 



Genus Adrana 

 Genus NUCULANA Link, 1807 

 Type species by monotypy. Area rostrata Chemnitz (^=Mya pernula 

 Miiller). Recent, seas of Europe. 



Shell with the posterior side narrowly elongate, arcuate, rostrate; an- 

 terior side shorter, higher and rounder. Rostrum more or less bicarinate, 

 subtruncate at the end which may show a slight gap, beaks small, opistho- 

 gyrate. Escutcheon distinct, elliptical, the lunule indistinct and small. Hinge 

 teeth numerous, arranged in two series, the posterior set almost twice as 

 long as the anterior. Chondrophore small, triangular, inclined backwards; 

 pallial sinus small, shallow. Sculpture of concentric threads, evenly spaced 

 on the umbo, crowded together ventrally. 



The genus Leda Schumacher, 1817 of the older writers has the same 

 type species. 



The genus Nuculana has been divided into many subgenera on small 

 differences, characters which cannot be easily expressed in a key. The fol- 

 lowing subgenera have species regional in the Panamic-Pacific Province. 



Subgenera of Nuculana 



I. Shell having the posterior side longer than the anterior. The posterior 

 set of hinge teeth nearly twice as long as the anterior. 



1. Posterior side with a double keel or bicarinate. Surface marked with 

 growth lines only. 



Subgenus Nuculana, s.s. 



2. Posterior side nodosely bicarinate. Surface with strong concentric 

 sculpture. 



Subgenus Costelloleda 



3. Posterior side much narrowed, attenuated and straight. Shape and 

 sculpture suggestive of a crassatellid. 



Subgenus Thestyleda 

 II. Shell with the anterior and posterior sides subequal in length, the an- 

 terior side high, inflated and rounded, the posterior side pointed at 

 the end. 

 A. Shells relatively small. 



4. Posterior side sharply rostrated, the rostrum concave or flattened, 

 bordered by an angle or ridge. 



Subgenus Saccella 



