303 



A single specimen was found by Dr. I). F. Lincoln, at Sec- 

 tion 1, Eighteen Mile Creek. It probably came from the 

 lower Naples shales. 



Class Crustacea. Lamarck. 



Order Ostracoda. Latr. 



The ostracods are small Crustacea, with a bivalve calcareous or horny 

 shell covering the entire body. The valves are joined dorsally by a 

 membrane, and open along the ventral side. The body is indistinctly 

 segmented, and bears seven pairs of appendages, two pairs of which 

 represent the trunk limbs. The shell corresponds to the carapace of the 

 higher crustaceans. These organisms are minute and will ordinarilj^ be 

 overlooked, imless search is made for them, with a lens, on the surfaces of 

 the shale laminte. They are especially abundant in the finer-grained 

 shales. 



Note. — The anatomy of modern Ostracoda should be studied with the 

 aid of the current text-books of zoology or anatomy. 



Genus PRDIITLl. Jo.xes and Hall. 



[Ety. : Primitia, first of the kind.] 

 (1865: Ann. and Mag. Nat. Hist., 3d Ser., Vol. XVI., p. 415.) 



Carapace minute with the valves equal, convex and 

 oblong ; hinge line straight ; surface of each valve impressed 

 dorsally, either at or anterior to the middle, h\ a vertical 

 sulcus of varialjle size. 



Primuha seminulum. Jones. (Fig. 242.) (Ann. and 

 Mag. Nat. Hist., Ser. V., Vol. XVH., p. 413, PL XIV.) 



Disting'uishin<>- ( 'harncters. — Convex, almost symmetrically 

 semi-circulai" outline; dorsal sulcus 

 almost central, extending across one- 

 third the width of the valve, or more; 

 surface clearly and elegantly reticu- 

 lated. 



Found in the Encrinal limestone, 

 at Eighteen Mile Creek, Lake Erie ^^^^ ^., p,..,„.,.„ ,,,„,,,„. 

 Shore. (Jones: Quart. Journ. Geol. ',^^^^1^^^!^^^^:^'"' "^^ 

 Soc, Vol. XLVL, p. 5.) 



