OSTRACODA. 343 



paratively colossal types, which range from one to three 

 inches in length. As a rule, also, the Palaeozoic Ostracoda 

 are plain, or are simply striated or granular ; whereas the 

 Mesozoic and Tertiary forms are commonly ornamented with 

 projecting tubercles, or in various other ways. As regards 

 their general distribution in time, the Ostracoda certainly 

 commence in the Upper Cambrian, and are even doubtfully 

 represented in the Lower division of this formation. They 

 existed under many and varied types in the Silurian, and 

 are abundant in all the succeeding formations, till the recent 

 period is reached. As matter of course, the remains of this 

 group of Crustaceans with which we have chiefly to do, are 

 principally those of the marine members of the order, and 

 this is especially true as regards the Palaeozoic species. In 

 later deposits, however, the occurrence of Ostracodes of types 

 now found in fresh or brackish water is by no means un- 

 common. 



The total number of fossil Ostracoda is very large, the 

 difficulties attending their study, for reasons already stated, 

 are quite exceptionally great ; and it is impossible, as a rule, 

 for the student to discriminate species, or, often, even genera, 

 unless he should have made the group a subject of special in- 

 vestigation. Here, therefore, no attempt will be made to give 

 even the briefest analysis of the families or genera of the 

 order ; but it may be well to shortly characterise some of 

 the common or more remarkable types, with special reference 

 to the Palaeozoic forms, with which, upon many grounds, it 

 is desirable that the student should have some acquaintance. 



Among the Palaeozoic Ostracodes, the first group that may 

 be noticed is that of which Leperditia (fig. 201, c and D) is 

 the type. In this genus the two valves are unequal in 

 size, smooth, nearly oblong, bean-shaped, with the posterior 

 end wider than the anterior. There is a small tubercular 

 eye-spot, placed on the head, near the hinge, and under- 

 neath and behind this is a slightly inflated area, correspond- 

 ing with an excavation of the shell interiorly, and exhibiting 

 reticulated or areolar muscular markings. Behind the eye- 

 spot is generally a vertical groove, which begins at the dorsal 

 margin, and extends a short way across the valves. The 



