4:20 PETRIFACTIONS AND THEIR TEACHINGS. CHAP, V. 



applied against the disk of the scale, forms the tube through which the 

 mucus flows. To this order belong the Mullet, Salmon, and Carp. Lign. 

 82, fig. 2, is the scale of a fossil Cycloid fish. 



FINS OF FISHES. As the progression of fishes through the 

 water is principally effected by the action of the tail, they 

 have no limbs properly so called. The instruments for 

 balancing the body, and for assisting progression, are the fins, 

 which are composed of numerous rays that support a mem- 

 branous expansion ; and the number and situation of the fins 

 present various modifications in the different orders and 

 genera. 



The spinous rays of the dorsal fins of the cartilaginous 

 fishes, as the Sharks and Rays, generally occur detached 

 from the body in a fossil state ; they are abundant in some 

 of the secondary deposits, and being often the only vestiges 

 of extinct species and genera, possess great geological in- 

 terest ; they are termed Ichthyodorulites (fossil dorsal-rays 

 offish}. 



TEETH OF FISHES. Of the durable parts of animals which 

 occur in the mineral kingdom, the teeth of Fishes present the 

 most numerous, varied, and striking modifications of form, 

 structure, composition, mode of arrangement, and attachment; 

 and yet these dental organs, separately considered, in many 

 instances fail to afford characters by which the natural affini- 

 ties of the original can be satisfactorily ascertained; and 

 without the aid of other parts of the skeleton, it is often im- 

 possible to determine whether an unknown form of tooth 

 belonged to an animal of the class of Fishes, or of Reptiles. 

 Although the modifications of form are almost innumerable, 

 they are referable to four principal types ; namely, the conical, 

 the flattened, the prismatic, and the cylindrical. 1 



STRUCTURE OF THE TAIL. The tail, as we have previously 

 mentioned, is the chief instrument of progressive motion in 

 these animals; it assumes two principal modifications; and 

 these characters the sagacity of M. Agassiz has invested with 

 a high degree of palseontological interest. 



In the greater number of the existing species, the vertebral 

 column terminates in a triangular plate of bone, to which the 



1 See " Medals of Creation," pp. 597601. 



