432 



PETKIFACTIONS AND THEIR TEACHINGS. CHAP. V. 



Mansfield, belonging to this genus of heterocercal sauroids, 

 which is remarkably fine. It is a large species, with small 

 scales, and the pectoral fin has a strong spine, or ray. Of 

 Acrolepis, a nearly related genus, there is an example from 

 the same locality at the bottom of this Case. 1 



A fine ichthyolite of the heterocercal sauroid genus, Platy- 

 gnathus, from Dura Den, is placed immediately above the last- 

 mentioned fossil. 



Megalichthys; Rhizodus; Saurichtliys. Wall-case B. (No. 

 9.) The specimens thus labelled are remains of three genera 

 of heterocercal sauroid fishes, from the carboniferous deposits 

 of Scotland, remarkable for the form and enormous size 

 of their teeth and jaws, and the strong enamelled plates of 

 the cranium. The conical, striated, gigantic teeth, some of 

 which are five or six inches in length, so closely resemble 

 those of crocodilian saurians, that they were generally 

 ascribed to reptiles, till M. Agassiz pointed out the true 

 characters of the originals. In Saurichthys the crown of 

 the tooth is separated from the fang by a constriction, and 

 the teeth are implanted in a furrow, or groove, as in the 

 Ichthyosaurus. Remains of several species are found in the 

 MuschelkalL 



The Megalichthys, a genus first made known by the elegant 

 Memoir on the carboniferous deposits of Burdie House, near 

 Edinburgh, by Dr. Hibbert, 2 in 183-4, has enormous conical, 



Den. 



Acrolepis. Keuper schist. 



Holoptychius: skull of un- Holoptychius Flemingii. 

 described species, from 

 Australia. Holoptychius Andersoni. 



In the " Trans. Eoyal Soc. Edinburgh/' vol. xiii. 



