630 THE PALEONTOLOGY OF MINNESOTA. 
[Ostracoda. 
also in nearly all the geological formations, and in some cases so numerously that 
whole layers are almost composed of their shells. The fossil forms are furthermore 
of great variety, since, with perhaps a single exception, all the families which have 
been established for the recent forms have also been recognized in the fossil state, 
while many types occur in the paleozoic rocks that seemingly are now totally extinct. 
With a single exception (Palwocypris), only the carapace valves are preserved in 
the fossil condition, and as these are often very similar in different genera and even 
families, it is evident that their study and classification is a matter of exceptional 
difficulty. To discriminate between these small fossils the paleontologist is obliged 
to rely on small differences in the shape, the relative size of the valves, the 
characters of the edges and of the hinge, the thickness of the valves, and the 
surface ornamentation. Among the paleozoic forms the valves are commonly lobed 
or sulcate and variations in these are usually counted important. A frequent 
difficulty is to distinguish between the anterior and posterior extremities. When 
not alike, the thickest end (it is generally also the highest) is considered as the poste- 
rior. It must be confessed, however, that this arbitrary determination can be 
accepted only as provisional. Some working rule like this is necessary until com- 
parison of other details of structure will have furnished us with more reliable criteria 
upon which to base conclusions. 
The carapace as stated consists of two calcareous or corneous valves of compact 
structure, commonly less than 4 mm. in length, though in a few cases the length 
exceeds 20 mm. The two valves may be equal (Tetradella, Primitia, etc.) or more or 
less unequal, with either the right or left overlapping the other at the ventral border 
only (Leperditia, Leperditella, etc.), or at the dorsal border as well (Bythocypris and 
Krausella), while in others the overlap is entire (Cytherella). The hinge or dorsal 
margin may be straight or arcuate, and, especially among the paleozoic types, is 
generally simple, though among more recent forms (Cytheridw) hinge teeth and 
corresponding sockets are not uncommonly developed. The anterior and posterior 
margins may be broadly or narrowly rounded, pointed or drawn out beak-like; and 
when the back is straight the ends may join it angularly. The ventral margin is 
oftenest convex though it is not infrequently straight or gently concave. The sides — e 
of the valves in the majority of Ostracoda may be said to be approximately even in 
contour or convexity but in many cases, especially among paleozoic forms, they are 
indented and thrown into two or more tubercles, lobes or ridges. The surface of 
the valves may be smooth and polished or it may be granulose, pitted, reticulose, 
striated, hirsute or otherwise marked, the effect being in many instances quite orna- 
mental. Finally many of the paleozoic Ostracoda of the family Beyrichiide have a 
wide, frill-like false border, which projects considerably beyond the true contact 
