FOSSIL FISHES. 55 



organs wliicli, under all ordinary circumstances, have disap})cared, viz., 

 the tail and tins. In some cases almost the perfect outlines of sharks six 

 or more feet in length are said have been found by the quarrymen 

 traced upon the surfaces of shale. One of these specimens represents the 

 heterocercal tail of a large shark in which the vertebrte have entirely 

 disappeared, leaving a smooth band representing the vertebral column. 

 On either side of this, however, the outlines of the iuterspinous bones 

 are distinctly traced ; but the most remarkable thing about this fossil is 

 that the lower lobe of the tail consists of rays that were distinctly ossi- 

 tied and now retain their original positions and forms. This indicates 

 that these old, Carboniferous sharks were as highl}^ organized as any oi 

 those inhabiting the present seas. 



'No similar instance of the preservation of the soft parts of cartilagi- 

 nous fishes is known to me except that of Chondrosfeus in the marls of 

 the Lias of England. In both these cases the unusual preservation of 

 the remains is probably due to the comparatively rapid deposition of an 

 earthy carbonaceous sediment over them. In the formation of lime- 

 stones, derived exclusively from organic structures, the accumulation of 

 material on the sea bottom must necessarily have been exceedingly slow, 

 and the soft parts of aquatic animals deposited with it have been so ex- 

 posed to decay, and to the depredations of the various forms of marine 

 life that subsist upon such food, that they have been totally destroyed. 



Formation and Locality : Black shale of Waverly Group, Vanceburg, Ky. 



Ctejtacanthus paevulus (n. sp.). 



Plate LIX., Fig. 3. 



Spine very small, 1 to 2 inches in length by 2 to 3 lines wide ; basal 

 portion relatively broad and long, flattened, iiTegularly striated ; exposed 

 portion slightly curved, acute, compressed, but arched transversely ; sur- 

 face covered with relatively broad, but somewhat irregular longitudinal 

 ridges. Posterior face set with large and much depressed hooks. 



This little spine is referred to Ctenacanthns with some doubt, as the 

 longitudinal ribs show no tubercles or scales such as are usually found in 

 the species of this genus. It agrees with them, however, in the gene- 

 ralities of its form and markings, and scarcely affords material for the 

 creation of a new genus. 



In the figure now given the longitudinal ribs are too strong and con- 

 tinuous. 



Formation and Locality : Cleveland shale, Bedford, Ohio. 



