46 LECTURK III. 



to establish three types of vertebra3, which may be characterised by 

 the form of the articular eucls of the centrum, as the " biconcave," 

 the " concavo-convex," and the " flattened" types, respectively dis- 

 tinguishing, as a general rule, Fishes, air-breathing Ovipara, and 

 Mammals. 



The least perfect form of a vertebra is that in fishes, though it 

 often seems the most complex, from the intercalation of bones of the 

 dermal system, and Geoffi'oy (ii. p. 119.)* was unfortunate in 

 taking a fish's vertebra, with this extrinsic complication, as the 

 perfect type of that primary segment of the myelencephalous ske- 

 leton. Two of the autogenous elements, the " hiemapophyses," for 

 example, are not developed till we reach the Reptilia : in fishes they 

 are represented by the " parapophyses " or lower transverse pro- 

 cesses. 



Before entering, however, upon the special osteology of the class 

 Pisces, it will be necessary to explain the sense in which the terms of 

 groups or divisions of the class are used in these Lectures, in reference 

 to their anatomical characters. Cuvier (xxiv. ii. p. 128.) primarily 

 divides the class, according to the nature of their endo-skeleton, into 

 Pisces ossei, and Pisces cartilaginei ; but the latter group includes 

 species of widely different grades of organisation, and in the " Tables 

 of Classification" exhibited in my first course of Lectures in this 

 Theatre in 1837, 1 separated the Lampreys, Myxinoids, and Lancelets, 

 under the name " De7-mopteri," from, the rest of the Chondroptertjgii 

 of Cuvier, making these the highest, and those the lowest order of 

 fishes, f 



M. Agassiz, with views enlarged by a survey of the extinct 

 members of the class, divided the Pisces, by characters taken from 

 the exo-skeleton, into four primary groups ; viz. Placoidei, Ganoidei, 

 Cycloidei, and Ctenoidei. 



The fishes of the Placoid order are characterised by having the 

 skin covered irregularly with plates of hard osseous matter, some- 

 times of large size, and sometimes reduced to small points, as where 

 they form the shagi'een on the skin of many sharks and the prickly 

 tubercles on the skin of most rays. This order comprehends all the 



* This ingeniows anatomist was thus driven to as arbitrary assumptions of mu- 

 tations of place, and to as far-fetched analogies, in reference to the supposed ele- 

 mentary parts of the vertebra, as in his attempt to exemplify the homologies of the 

 cephalic division of the endo-skeleton of the higher Vertebrata, by the combined 

 bones of the exo- and endo-skeleton, which constitute the complex skull in 

 Fishes. 



■j- The distinguished naturalist, C. L. Bonaparte, Prince of Canino, has also 

 founded a distinct order for the Cyclostomous Chondropterygians of Cuvier, under 

 the name of " Marsipo-branchii," which well applies to the Lampreys and Myxi- 

 noids, — Selachorum Tabula Analytica, 1838. 



