448 



PETRIFACTIONS AND THEIR TEACHINGS. 



CHAP. V. 



PLACOID ORDER. In the placoid fishes the skeleton is cartilaginous, 

 the gills are fixed, the skin is not covered with scales as in the other 

 three orders, but either studded with bony tubercles or scutcheons of 

 enamelled bone, or protected by very small bristly plates, constituting a 

 tesselated integument, called shagreen. In consequence of the perishable 

 nature of the skeleton, the teeth, spines, or fin-rays, tubercles and 

 scutcheons, vertebrae, and in some rare instances the dermal shagreen, are 

 the only parts preserved in a fossil state. The Placoids are the most 

 ancient animals of the vertebrated classes hitherto discovered, for rays 

 of a species of shark (Onchus) have been found in the Lower Silurian 

 deposits ; and they have continued through the entire series of forma- 

 tions, and abound in the present seas. But though the fishes of this 

 order are the most universally distributed in time as well as in space, 

 the relative numerical predominance of the several families varied 

 greatly in different periods. 



In Table-Cases 2, 4, 6, 8, 10, the collection of teeth, vertebrae, fin-rays, 

 or Ichthyodorulites, &c. are arranged under the respective families to 

 which they belong. 



CHIM^EROIDS. Table-Case 4. A good collection of the maxillary 



organs, and the hard in- 

 divisible plates of dentine, 

 composing the dental in- 

 struments of this curious 

 tribe of the Shark family, 

 belonging to four sub- 

 genera, is deposited in 

 Table-Case 4. They are 

 referable to Ischyodus, 

 Edaphodon, Psittaffodon, 

 and Ceratodus. Among 

 them are beautiful ex- 

 amples from the Eocene 

 clay of Bracklesham, in 

 Sussex, from Mr. Dixon's 

 collection, and figured and described in his work by Sir Philip Egerton. 

 There is one pair of mandibles in a block of chalk, remarkable for their 

 prolongated and curved form, which led M. Agassiz to name the genus 

 Psittacodon ; the Edaphodon, of Sir P. Egerton. 1 



SQUALID^:, or SHARKS. The fossil teeth of this universally distributed 

 family of voracious fishes, are abundant in almost every secondary and 

 tertiary deposit. Want of space compels me to refer to " The Medals 

 of Creation," for a popular account of these fossil relics. 2 The collection 

 is very rich in the usual types of the genera and subgenera. The sharp 

 triangular teeth, with or without lateral denticles, and the cutting 

 edges, either smooth or serrated, occur in profusion in many tertiary 

 strata, and species of the same genera abound in the chalk. (Lign. 96.) 



HYBODONS. A family allied to the Sharks, but with conical and un- 

 compressed teeth. These fishes had two dorsal fins, with anterior 



LIGN. 95. MANDIBLE OF A CHIMJEROID FISH 



EDAPHODON MANTELLI, FROM THE CHALK, 



LEWES. ( nat. size.) 



See " Geological Journal," May 1847. 



2 Chap XV. p. 611. 



