418 PETRIFACTIONS AND THEIR TEACHINGS. CHAP. V. 



consist of the mineralized osseous skeleton. Deposits of mud 

 or fine detritus, of whatever age, appear to have been most 

 favourable for the preservation of the dermal integuments ; 

 hence we often find in the pulverulent clays and marls of the 

 Tertiary strata, and in the Chalk of England and Westphalia, 

 and in the fine lithographic stone of Solenhofen, fishes, perfect 

 in form, and not only individuals, but groups, with the scales, 

 fins, head, teeth, and even the capsule of the eye, in their 

 original juxtaposition. In Chalk, some of the fishes occur 

 with the body uncompressed, and as entire, as if the original 

 had been surrounded by soft plaster of Paris while float- 

 ing in the water. But in coarse limestones and conglome- 

 rates in other words, in materials that have been subjected 

 to the action of the waves and torrents detached teeth, 

 scales, bones, &c. constitute the principal vestiges of this class 

 of beings. 1 



The cartilaginous or osseous nature of the skeleton, and the 

 number and position of the fins, were the characters formerly 

 employed in the classification of Fishes ; but M. Agassiz, con- 

 ceiving the structure of the skin to afford a natural index to 

 the essential modifications of organization and functions, with 

 great sagacity adopted an arrangement founded upon the 

 form and structure of the scales, and divided the whole class 

 into four orders, each distinguished by essential differences in 

 the dermal (skin) system. 



To the geologist this method has proved of inestimable 

 value ; for it is simple, easy of application, and, so far as our 

 present knowledge extends, may be relied upon as affording 

 accurate conclusions as to the nature and relations of the 

 originals, to which a few detached fossil scales may have 

 belonged. Another important aid has been derived from the 

 microscopical examination of the structure of the teeth ; a 

 department of palseontological investigation, which is yet but 

 partially explored. 



The living species of Fishes exceed eight thousand, and 

 those found in a fossil state, and determined by M. Agassiz, 

 already amount to upwards of two thousand, while several 

 hundreds are still undescribed, and the rapid progress of 

 geological research is continually adding to the number. The 



1 See " Medals of Creation," chap. xv. 



