114 PROVISIONAL AID TO THE STUDY OF TASMANIAN MOLLUSCA. 



margins crenated inside. Dist. — West Indies,,. 

 Britain, India, New Zealand, West America ; ranging 

 from 8 to 60, rarely 120 fathoms. 



Genus Limopsis. (685.) 

 Shell orbicular, convex, slightly oblique, ligamental area with 

 a triangular cartilage pit in the centre ; hinge with two 

 equal curved series of transverse teeth. Dist. — Eed 

 Sea (Nyst.), Japan, Britain, Tasmania. Mr. M' Andrew 

 has dredged L. Pygmaa living on the coast of Finmark. 



(xciv.) Family Mytilidjb. 

 Shell equivalve, oval or elongated, closed, umbones anterior,,. 

 epidermis thick and dark, often filamentose ; ligament 

 internal, sub-marginal, very long; hinge edentulous;. 

 outer shell-layer obscurely prismatic-cellular ; inner 

 more or less nacreous ; pallial line simple ; anterior 

 muscular impression small and narrow ; posterior 

 large, obscure. 



Genus Mttilus. (686-691.) 

 Shell wedge-shaped, rounded behind, smooth in the typical 

 species ; umbones terminal, pointed ; hinge-teeth 

 minute or obsolete ; pedal muscular impressions two in 

 each valve, small, simple, close to the adductors. 

 Dist — World-wide, Ochotsk, Behring's Sea, Russian 

 Ice-meer, Black Sea, Cape Horn, Australasia. 



Genus Modiola. (692-694.) 

 Shell oblong, inflated in front ; umbones anterior, obtuse ; 

 hinge toothless ; pedal impressions three in each valve, 

 the central elongated ; epidermis often produced into 

 long beard-like fringes. Dist. — Universal. 



Genus Modiolaeia. (695.) 

 Shell rhomboidal, sculptured by two rows of strise (one on 

 each side), which radiate from the beaks, leaving the 

 middle portion smooth ; umbones incurved ; hinge 

 edentulous or crenulated, hinge-plate finely notched. 

 Dist. — Temperate and Arctic Seas. 



(xcv.) Family Aviculidj3. 

 Shell inequivalve, very oblique, resting on the smaller (right) 

 valve, and attached by a byssus ; epidermis indistinct; 

 outer layer prismatic-cellular, interior nacreous ; 

 posterior muscular impression large, sub-central, 

 anterior small within the umbo; pallial line irregularly 



