630 THE PALEONTOLOGY OF MINNESOTA. iostncod 



also in nearly all the geological formations, and in some cases so numerously that 

 wholg 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 (Paheocypris), 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, Primiiia, etc.) or more or 

 less unequal, with either the right or left overlapping the other at the ventral border 

 only (Leperditia, Leper ditel la, etc.), or at the dorsal border as well (Bythocypris and 

 Krausella), while in others the overlap is entire (Cytherelld). The hinge or dorsal 

 margin may be straight or arcuate, and, especially among the paleozoic types, is 

 generally simple, though among more recent forms (Cytheridce) 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 

 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 Beyrichiidce have a 

 wide, frill-like false border, which projects considerably beyond the true contact 



