SUPER-ORDER III OSTRACODA 643 
smooth and glossy, or granulose, pitted, reticulose, striate, hirsute, or other- 
wise marked, the effect being often quite ornamental. The two valves may 
be of equal size (Beyrichia), or more or less unequal, with either the right or 
left valve overlapping at the ventral border only (Leperditia), or at the dorsal 
border as well (Bairdia), or in some cases overlapping all round (Cytherellu). 
Most commonly the outline is ovate or reniform ; in many cases, however, 
one or both ends may be pointed or drawn out in the form of a beak; and 
when the dorsum is straight, the ends may join it angularly. Although 
usually convex, the ventral margin is sometimes straight or gently concave. 
It is sometimes impossible to distinguish between anterior and posterior 
extremities, but as arule the posterior half is somewhat thicker than the 
other, even though of equal or of lesser height. The hinge line may be 
straight or arcuate, the hinge itself being generally simple, although among 
the Cytheridae hinge teeth and corresponding sockets are often developed. 
There are commonly a small median and two larger lateral eyes ; the position 
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 leading 
formations, and 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. An arrangement of 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 considerable size. Valves smooth and glossy, of 
very compact structure, and in general regularly conver ; hinge line straight ; antertor 
and posterior ends obliquely truncated or rounded, and neither gaping nor excised. 
Leperditia, Rouault (Fig. 1337). Shell sub-oblong with an oblique backward 
swing, from 2 mm. to 22 
mm. long; dorsal edge 
straight, generally an- 
gular at the extremities ; 
ventral outline rounded. 
Valves unequal, the 

right larger and overlap- Fic. 1338. 
ping ventral edge of the Fic. 1837. Isochilina gigantea, Roemer. 
in | tybbe e often Leperditia Hisingeri, Schmidt. Silurian ; Silurian erratic; Lyck, East 
left urfac te Wisby, Gottland. I). "Prussia. 2/3 (after F. Roemer). 
corneous in appearance, 
smooth, and eye tubercle generally present on the antero-dorsal quarter. A large 
rounded sub-central muscular imprint present on interior, Ordovician to Carboniferous. 
Leperditelia, Ulrich. Similar to above, but the left instead of right valve is the 
larger, and has a groove within its ventral border for receiving simple edge of the 
right. Eye tubercle wanting. Length 1 mm. to 3 mm. Ordovician. 
Isochilina, Jones (Fig. 1338). Like Leperditia except that the valves do not over- 
lap but are equal in every respect. Ordovician and Silurian. 
