164 Mr, C. Davies Sherborn's notes m the 



as they are called, often take in particular genera a definite and 

 recognisable grouping ; and, as they are remarkably persistent 

 in the different species, form a not unimportant consideration in 

 the determination of the genus. 



In addition to the elastic ligament, which serves to bind 

 together the two valves, many of the Ostracoda are provided 

 with hinge-teeth. In the genus Cythere there is usually a strong 

 tooth at each dorsal angle in the one valve ; these teeth are 

 joined by a groove, and fit into corresponding hollows in the 

 other valve ; these hollows are connected by a bar or ridge, 

 which in its turn fits into the groove of the toothed valve. 

 The large angle teeth of Cythere frequently consist of three or 

 more denticles, and this seems to be a characteristic feature of 

 the Jurassic members of the gi-oup. In the genus Cijtheridea there 

 is a series of denticles or crenulations along the whole of the 

 dorsal margin in one valve, with a corresponding series of small 

 pits in the other valve, the bar and furrow being, of course, 

 absent. In Bythocythere the hingement consists of a simple bar 

 and furrow. In CypHs the hingement is more simple, an over- 

 lapping flange. Candona, Aglaia, Argillcecia, and other Cypridida 

 have simple contact margins ; while in the genus Cytherella one 

 valve is larger as a rule than the other, the smaller of the two 

 fitting inside the larger with a rabbet edge, similar to a shouldered 

 pill-box. In Bairdia the hingement is flexed but simple, the left 

 valve being much larger than the right, and overlapping all 

 round the margins. In Cythere and some other genera the 

 hinges can be seen at the angles outside the shell. 



The ventral margin is usually unbroken, except in rare cases 

 by a few serrations ; but in the genera Cypridina, Cypridea, 

 Asterope, &c., a distmct and sometimes deeply cut notch is seen 

 in the antero -ventral margin. 



The junction of the ventral margins of the valves in the 

 Ostracoda also shows a variety of conditions, though usually 

 more regular than those of the dorsal. They may be flush as in 

 many Cythera, but it is more usual to find one of the valves over- 

 lapping the other slightly in the middle of the margin. This 

 overlap sometimes becomes very strong, and in such genera as 

 Bairdia and Pontocypris forms a well-developed roU. Usually 

 the valves close anteriorly and posteriorly, but in some species, 

 Asterope teres for example, there is a slight gape at the anterior 

 end. 



With regard to the external ornament of the valves, some of 

 the Ostracoda, notably the Cytherida, are very richly sculptured 

 and decorated with fringes, spines, ridges, or tubercles ; and 

 amongst the fossil forms specific determinations are based very 

 largely upon this external sculpturing. Shell surface-marking 

 is well known to be a very variable character, and we find in 



