VERTEBRATES. 13 



smart blow of the hammer, so that the delicate fossils which it 

 contains are often obtained entirely detached from the matrix. 

 Although only about ten feet square of surface was exposed at 

 this locality, it has afforded more than a thousand well pre- 

 served teeth. So thickly imbedded were they in this single 

 layer, that a hand specimen of the rock, not exceeding four 

 inches square, often exhibits a half dozen or more perfect teeth. 

 The associated fossils are Produetus punctatus, Actinocrinus 

 Naslivillce, Zaphrentis Dalii and Sphenopoterium obtusum. 



The third fish bed in the ascending order is found in the ] 

 upper part of the Keokuk limestone, just below the base of the / _ y^J 



geode bed. It was first observed in the vicinity of Warsaw, / 2 «^sL-^ /^ 

 and subsequently identified at Nauvoo and some other locali- I ' 



j_^ "\ ties in Hancock county. It is characterized by the remarkably 

 large palate teeth of Cochliodus nobilis (N. and W.) , and the large I 

 spine, Drepanacanthus gemmatus. Like the beds before de- 

 scribed this consists of a single stratum of brownish-gray lime- 

 stone, not exceeding four or five inches in thickness, and 

 \ contains, besides the remains of fishes, Spirifer neglectus, S. 

 ^- Keokuk, Productus punctatus and Zaphrentis Dalii. 

 ( Ascending in the series to the horizon of the St. Louis lime- \ 

 stone, the teeth of fishes become comparatively rare, and are \ 

 <*-" ,J only found as isolated specimens, sparingly dispersed through j~^ 



ls~ the rocky strata. The quarries in the vicinity of Alton and / 



St. Louis, and near Waterloo, in Monroe county, have afforded 

 V a few fine species. 



The Upper Archimedes or Chester limestone is more prolific 

 in ichthyic fossils than the beds last named, and we again meet 

 with strata in this group where they are crowded into a limited V 

 space in great numbers. The upper fish bed is found at the 

 junction of the lower limestone in the Chester group, with the 

 green shales above, and the stratum containing these fossils 

 consists of thin plates of limestone imbedded in the shale. 

 The rock is usually arenaceous and sometimes highly ferru- 



