834 PELECYPODA. [Protobranchia. 



narrowly arched toward the base. Lunule elongated, not very dis- 

 tinctly marked off. Escutcheon very distinct, elliptical, bounded by 

 a rounded ridge on either side. Sculpture consisting of obscure con- 

 centric sulcations and microscopic radiate striae. Epidermis thin, 

 polished, of a golden hue. Colour yellowish-green, lighter at the 

 umbones. Interior pearly, microscopically radially striate ; margins 

 smooth, sharp. Hinge with a long and oblique narrowly triangular 

 resiliurn ; the two series of teeth not very high, sharp, 7 teeth on the 

 posterior and 11 on the anterior side, highest in the middle of the 

 row. Adductor-scars not deep, subequal. P allied line simple, some- 

 what uneven. 



Length, 10 mm. ; height, 7 mm. ; diameter, 5 mm. 



Type in the British Museum. 



Hob. Near Tiri-Tiri Island, in 20 fathoms (Captain Bollons) ; 

 Lyttelton Harbour, in 3 fathoms (H. S.) ; Stewart Island, in 15 fathoms 

 (A. Hamilton) ; Cook Strait. Also Australia. 



Fam. LEDID^l, Adams. 



Animal : The same characters as the Nuculidce, but the mantle 

 has 2 posterior sutures and 2 united siphons ; the heart traversed 

 by the rectum ; pallial line usually sinuated. 



Shell as in the Nuculidce, but elongated, with the ligament variable, 

 the resilium sometimes external or absent, the internal shell layer 

 subnacreous or porcellanous, the ends of the shell partly gaping. 



Distribution. All seas. 



Silurian to Recent. 



KEY TO GENERA. 



A. Ligament external. 



a. Ligament prominent, large, elongated, with 2 series of 



V-shaped teeth . . . . MALLETIA. 



aa. Ligament minute, often hidden, with 1 posterior lateral 



tooth and about 6 cardinal teeth . . . . . . PLEURODOK. 



B. Ligament internal (resilium). 



a. Shell beaked and carinated posteriorly ; all teeth V-shaped LEDA. 

 aa. Shell not beaked and not carinated posteriorly ; posterior 



teeth straight, elongate, imbricating . . POROLEDA. 



Genus 1. LEDA, Schumacher, 1817. 



Leda, Schumacher, Essai Nouv. Syst. Habit. Testaces, 1817, 173. Type : 

 MI/II pernula, Miiller. Nuculana, Adams, Meek, and others ; not of Link. 



, 'Animal having the mantle-borders produced posteriorly into 2 

 lobes which simulate a third siphon. The siphons are short and 

 united ; labial palps large, with posterior appendicles ; branchiae 

 narrow ; foot large, lanceolate in front, and forming a disc with crenu- 

 lated margin. There is no byssus. 



Shell solid, transversely oval, more or less beaked behind ; umbones 

 somewhat turned backward ; surface concentrically and obliquely 

 furrowed, with a posterior ridge or carina ; hinge-teeth numerous, 



