278 



PALAEONTOLOGY. 



of the seven fossil genera of this family have any living 

 representative. 



FIG. 189. Teeth of Lamna. 



(a & d) Teeth of Lamna, agreeing in form with species abundant in the London clay 



and red crag. 

 (5) Tooth probably of the same genus, but of an undescribed species, provided with 



quadrate lateral denticles, 

 (c) Tooth of Lamna with two pairs of denticles. 

 (e) A tooth, of which the form probably depends upon its situation having been near 



the termination of the series. 



ICHTHTODOEFLITES are the spines of the dorsal fins of 

 placoid fishes. On the form and structure of these singular 

 bodies, several new genera have been established. 



The Order GANOID, Agass. Have the body enclosed in a 

 coat of mail formed of the scales. These are composed of 

 two substances an internal horny or osseous lamina, and an 

 external layer of hard enamel, brilliant and polished like the 

 surface of the teeth. The scales are generally of an angular 

 or rhomboidal form ; they are regularly united by their 

 margins, and lock into each other in a singular and compli- 

 cated manner, by a spine and socket. 



The bones of the skeleton are more dense than in the 

 placoids, but they are not so completely ossified as in the 

 other orders. In the Sturgeon the skeleton is cartilaginous. 



1st Family. The ACIPENSERIDJE, of which the Sturgeon 

 is the type, have the body protected by ranges of dermal 

 plates. We know two genera. A fossil species of Acipenser 



