SUBCLASS 11 



EUCRUSTACEA— 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, Mi/odocopa and Cladocopa, 

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

 only the more repx'esentative genera can be noticed. 



Family 1. Leperditiidae Jones. 



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

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

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



Shell sub-oblong with an oblique backward 



dorsal edge straight, generally angular at the 



Valves unequal, the right larger and over- 



Leperditia Rouavxlt (Fig. 1423). 

 swing, from 2 mm. to 22 mm. long; 

 extremities ; ventral outline rounded, 

 lapping ventral edge of 

 the left. Surface often 

 corneous in apjjearance, 

 smooth, and eye tubercle 

 generally present on 

 the antero-dorsal 

 quarter. A large 

 rounded sub - central 

 muscular imprint pre- 

 sent on interior. Ordo- 

 vician to Carboniferous. 



Leper ditella 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. 



Aparchites 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. 



Fig. U-23. 



Leperditia hisingeri Schmidt. Silurian ; 

 Wisby, Gotland, i/i- 



Fir.. 1424. 



Isochilina gigantea Roenier. 

 Silurian eriatic ; Lyck, East 

 Prussia. 2/3 (after F. Roemer). 



Family 2. Beyrichiidae Jones. 



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

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

 which the surface of the valves is raised into numerous low lobes, 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 



