SUBCLASS II EÜCRUSTACEA— OSTRACODA 737 



of the latter being often indicated on the exterior of the valves by a small 

 " eye tubercle," or ocular spot. 



Save for one or two families (Cypridae) Ostracods are almost wholly 

 restricted to marine or brackish water. They are gregarious, and occur in 

 vast hordes swimming near the surface or creeping over the bottom, preferring 

 usually shallow depths. Their remains abound in nearly all the principal 

 formations, and they are often important rock-builders. The identification of 

 fossil Ostracods is very difficult on account of their similarity of form and 

 ornamentation, and usually minute size ; and they cannot be well intercalated 

 among the recent series for reasons already given. Sars has arranged the 

 living forms into four divisions, Podocopa, Platycopa, Myodocopa and Cladocopa, 

 but assembling the families into higher groups is not attempted here, and 

 only the more representative genera can be noticed. 



Family 1. Leperditiidae Jones. 



Thick-shelled Ostracoda, mostly of considerahle size. Valves smooth and glossy, of 

 very compact structure, and in general regularly convex ; hinge-line straight ; anterior 

 and posterior ends ohliquely truncated or rounded, and neither gaping nor excised. 



Leperditia Rouault (Fig. 1423). Shell sub-oblong with an oblique backward 

 Swing, from 2 mm. to 22 mm. long; dorsal edge straight, generally angular at the 

 extremities ; ventral outline rounded. Valves unequal, the right larger and over- 

 lapping ventral edge of 

 the left. Surface often 

 corneous in appearance, 

 smooth, and eye tubercle 

 generally present on 

 the antero-dorsal 



quarter. A large 



rounded sub - central p ,^23 ^^°' ^^^^' 



1 • • . ■ ■ Inochilina gigantea Roemer. 



muSCUiar impnnt pre- Leperditia Usingen Schmidt. Silurian ; Silurian erratic ; Lyck, East 



sent on interior. Ordo- Wisby", Gotland. i/i- Prussia. • 2/3 (after F. Roemer). 



vician to Carboniferous. 



Leperditella Ulrich. Similar to above, but the left instead of right valve is the 

 larger, and has a groove within its ventral border for receiving the simple edge of 

 the right valve. Eye tubercle wanting. Length 1 mm. to 3 mm. Ordovician. 



Isochilina Jones (Fig. 1424). Like Leperditia except that the valves do not over- 

 lap but are equal in every respect. Ordovician and Silurian. 



Äparchites Jones. Shell not over 3 mm. in length, equivalve, sub-ovate or 

 oblong ; ventral edge thickened, often bevelled. Ordovician and Silurian. 



Schmidtella Ulrich. Ordovician. Paraparchites Ulrich and Bassler. Carboni- 

 ferous : North America. 



Hfefe^l^B H^^^p 



Family 2. Beyrichiidae Jones. 



Small equivalve Ostracoda with a long straight hinge. Shells vertically sulcated and 

 more or less lohate, varying from forms having a simple median depression to oihers in 

 ivhich the surface of the valves is raised into numerous low lohes^ ridges or nodes. 



Primitiella Ulr. (Fig. 1425, a). Valves with a broad, undefined mesial depression 

 in the dorsal slope. Ordovician to Devonian. 



VOL. I 3 B 



