536 THE WONDERS OF GEOLOGY. Lect. V. 



and the debris of fishes and reptiles. Aust Cliff, on the 

 Severn, near Westbury, in Somersetshire, is a well-known 

 locality for the fossils of this remarkable deposit. By 

 some geologists this bone-bed is included in the Trias, 

 and by others in the Lias. A similar stratum occupies 

 the same geological position in Germany, and contains 

 organic remains of a like nature ; specimens of teeth of 

 fishes of the genus Ceratodus (Med. Creat. p. 618), are 

 very frequent in this bed. 



Among the Ichthyolites that prevail in the Lias, the 

 scales and teeth of a genus of Ganoids called Dapedius 

 (Licjn. 120), are especially abundant, and entire specimens 

 of the fish are often met with. I am led to notice this genus 

 more particularly, that your attention may be directed to a 

 remarkable modification of structure observable in the 

 Ichthyolites of the more ancient deposits, which will here- 

 after come under our examination. The Dapedius belongs 

 to that division of the Lepidoids in which the tail is homo- 

 cereal, or equal-tailed ; while in all the genera of the 

 palaeozoic strata (p. 203) the tail is heterocercal, or unequal- 

 lobed.* But few of the existing genera of fishes have this 

 condition of the caudal fin, while it is found in almost every 

 fossil species below the Triassic deposits.f 



31. Reptiles of the Jurassic system. — The preva- 

 lence of animals of the reptile class during the Oolitic 

 era, was incidentally alluded to in the course of our 

 previous observations on the organic contents of the 

 different members of this extensive formation. From 

 the lowermost stratum, the bone-bed of the Lias, to the 

 uppermost layer of the Portland stone, the remains of 



* See Medals of Creation, p. 601. 



f It is worthy of remark, that in the embryotic state the tail in all 

 fishes is heterocercal, and passes into the homocercal type in the pro- 

 gress of development in those species which have this form of the 

 caudal appendage when arrived at maturity. 



