505 CRUSTACEA. 





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 uncommon. 



The total number of fossil Ostracoda is very large, the difficulties 

 attending their study, for reasons already stated, are quite excep- 

 tionally great ; and it is impossible, in many cases, for the student 

 to discriminate species, or, often, even genera, unless he should have 

 made the group a subject of special investigation. 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 char- 

 acterise 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. 361, c and d) is the type 

 (Leperditidce). 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 underneath and behind this is a 

 slightly inflated area, corresponding 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 

 genus ranges from the Ordovician to the Carboniferous. Isochilina 

 (fig. 361, b) nearly resembles the preceding, but the valves are 

 equal. It is not uncommon in the Ordovician rocks. 



The genus Primitia (fig. 361, e-g) comprises another group of 

 Palaeozoic Ostracodes, which is essentially characteristic of the 

 Cambrian, Ordovician, and Silurian deposits, though apparently 

 also represented in the Carboniferous. In this genus the carapace 

 is equivalve, convex, and oblong in shape, indented with a vertical 

 dorsal groove of variable depth. Related to Primitia is the familiar 

 genus Beyrichia (fig. 361, h and 1), which is confined to the Cam- 

 brian, Ordovician, and Silurian rocks, and which sometimes is pres- 

 ent in such vast numbers as to give rise, by the accumulation of the 

 carapace-valves, to regular bands of limestone. The carapace of 

 Beyrichia is more particularly distinguished by the possession of 

 two or three transverse grooves, which start at the hinge, and pass 

 partially or wholly across the valves. In the curious Beyrichia (?) 

 oculifera, the eye-spot forms a prominent and faceted tubercle, un- 

 like that of any other Ostracode (fig. 361, 1). Two other members 

 of the ancient family of the Leperditidce may be just mentioned — 

 namely, Kirkbya and Moorea, of which the former ranges from the 

 Silurian to the Permian, while the latter is principally Carboniferous, 

 and is doubtfully represented in the Jurassic rocks. 



