106 PROVISIONAL AID TO THE STUDY OP TASMANIAN MOIXUSCA. 



Genus Mtochama. (584-585.) 

 Shell inequivalve, attached by the dextral valve, and modified by 

 the form of the surface of attachment ; posterior side 

 attenuated; left valve gibbose ; cartilage internal, between 

 two tooth-like projections in each valve, and furnished 

 with a movable ossicle ; anterior muscular impression 

 curved, posterior rounded, pallial sinus small. Dist. — 

 Australasia ; attached to Crassatella and Trigonia in 8 

 fathoms water ; the fry, (as indicated by the umbones) 

 is free, regular and Myodora-shaped. 



(lxxvi.) Family Macteidje. 

 Shell equivalve, trigotial, close or slightly gaping ; ligament 

 (cartilage) internal, sometimes external, contained in a 

 deep triangular pit ; epidermis thick; binge with two 

 diverging cardinal teeth, and usually with anterior and 

 posterior laterals ; pallial sinus short, rounded. 



Genus Mactea. (586-588.) 

 Shell nearly equilateral ; anterior hinge-tooth j± shaped, with 

 sometimes a small laminar tooth close to it ; lateral 

 tooth doubled in the right valve. Dist. — All seas, 

 especially within the tropics ; 35 fathoms. 



Genus Luteaeia. (589.) 

 Shell oblong, gaping at both ends ; cartilage plate prominent, 

 with 1 or 2 small teeth in front of it in each valve ; 

 pallial sinus deep, horizontal. Dist. — United States, 

 Brazil. Britain, Mediterranean, Senegal, Cape, India, 

 New Zealand, Sitka. 



(lxxvii.) Family PAPHiiDiE. 

 Genus Pahia. (590-595.) 

 Shell trigonal, thick, compressed, closed ; ligament internal, in a 

 deep, central pit ; a minute interior hinge-tooth, and 

 1 — 1 lateral teeth in each valve ; muscular scars deep ; 

 pallial sinus small. Dist. — West Indies, Mediterranean, 

 Crimea, India, New Zealand, Tasmania, Chili ; sands 

 at low-water. 



Sub-Genus Mesodesma. Desh. 

 Shell oval, sub-equilateral; lateral teeth short, smooth, sub- 

 equal ; siphonal inflection distinct. 



Sub-Genus Donacilla. Lam. 

 Shell elongate, cuneiform, slightly truncate posteriorly ; anterior 

 lateral teeth elongated; posterior short ; siphonal sinus 

 distinct. 



