126 PE0VISI0NAL AID TO THE STUDY OF TASMANIAN MOLLTTSCA. 



HlNGE-LINE AND TEETH. 



The dorsal margin on which the liga- 

 ment and teeth are situated is 

 termed the hinge-line. It is long 

 and straight in Area, short in 

 Vulsella, and curved in most 

 genera. 



The central teeth immediately under 

 umbo are called hinge or cardinal 

 teeth (iS 3 -), those on each side are 

 lateral teeth (48 b ), either the 

 cardinal or the lateral teeth may- 

 be lacking in many shells, while 

 in some teeth are entirely absent. 



Edentulous. 

 The dentition formulae are usually stated 

 as follows : — Cardinal teeth 2-3 or 

 | — meaning 2 in the right valve, 3 

 in the left; lateral teeth, 1 — 1, 

 2 — 2, or 1 anterior and 1 posterior 

 in right valve, 2 anterior, and 2 

 posterior lateral teeth in the left 

 valve. 



Adductoe oe Muscular Isipbessions. 

 The greater number of bivalves have 

 two adductor muscles, whose im- 

 pressions or scars on the shell, 

 nearly equal in size, are situated 

 respectively on the anterior and 

 posterior sides at the extremities 

 of the hinge-line. In Mytilus, 

 Modiola, these two adductor im- 

 pressions are unequal, the pos- 

 terior one being very much larger. 

 In the Pectens, Oysters andLimas, 

 there is only one strong adductor 

 impression (53 !l ), placed more 

 centrally. The single muscular 

 scar is not quite central, but 

 nearer the posterior than the 

 anterior side. 



Epideemis. Scaep oe Skin. 

 All bivalve shells are clothed with an 

 epidermis, scarf, or shin. Incon- 

 spicuous in some, but remarkably 

 developed in others. 



Example i 



No. 



(49 '■«•) 



