Moittaciita.} PELECYPODA. 915 



Distribution. European seas, coasts of North America, Australasia. 

 The type is from deep water on the Devonshire coast. 

 Fossil in the Tertiary. 



1. Montacuta triquetra, n. sp. Plate 53, figs. 7, n. 



Shell very small, thin and fragile, subtrigonal. truncated behind, 

 inequilateral, smooth, white. Beaks not prominent, contiguous, small, 

 pointed. Anterior end produced, sharply rounded, the dorsal margin 

 descending, straight ; posterior end shorter, slightly angled at the 

 middle and truncated below, the straight dorsal margin descending, 

 basal margin faintly convex. An elongated lunular area is slightly 

 indicated. Sculpture consisting of very fine concentric strise only. 

 Colour white. Interior white, smooth. Margins simple, thin and sharp. 

 Hinge-line very narrow ; right valve with an anterior long and narrow 

 lamina and a well-pronounced cardinal tooth, which is narrowly tri- 

 angular and situate just in front of the beak ; left valve with a broadly 

 elevated, sometimes obsolete, cardinal tooth in front of the umbo. 

 Resilium posterior, rather long and narrow, the nymph of the right 

 valve better developed than that in the left valve. Adductor-scars 

 subequal, within the pallial line, which is continuous and simple. 



Diameter Ant. -post., 3-7 mm. ; dorso-ventral. 3-3 mm. : thick- 

 ness, 2 mm. 



Type in my collection. 



Hob. Port Pegasus, Stewart Island, in 18 fathoms (Captain 

 Bollons). 



Fam. DIPLODONTID.E;, Daii. 



Animal with the external limb of the gills developed, reflected, 

 and sometimes appendiculate, the adductor muscles not projecting 

 into the discs of the mantle, without siphons, the foot long, vermiform. 



Shell subcircular in outline, rarely nestling and irregular ; hinge 

 with the laterals obscure or absent, valve-margins plain ; the adductor- 

 scars continuous peripherally with the pallial line. 



Cretaceous to Recent. 



Genus 1. DIPLODONTA, Bronn, 1831. 



Diplodonta, Bronn, Italians Tertiar Gebilde, 1831, p. ix. Type : Venus 

 lupinus, Brocchi ; not Diplodon, Spix, 1827. Mysia, Brown, 1833. 

 Egeria, Lea (in part) ; not of Roissy, 1805. Sphcerella, Conrad, 1838. 

 Glocomene, Leach, 1852. Cycladicama, Valenciennes, 1854. Mittrea, 

 Gray, 1857. 



Animal with double gills, without siphons, the generative and 

 digestive glands contained within the general mass of the body. Foot 

 long, vermiform, with a terminal glandiform swelling, not perforated, 

 and without a byssus gland ; labial palps moderately elongated, sub- 

 trigonal. 



Shell rotund, equilateral, externally concentrically striated or 

 smooth, with inconspicuous epidermis ; 2 cardinal teeth in each 



