STURlONIDiE 1 45 



have been discovered in the chalk formations of Kent and 



Sussex. They have been referred by Agassiz to 



the genus called Maeropoma, significative of the 



large size of the gill-cover, and to the coelacanthal 



family. Casts of the " interior" of the alimentary 



canal, showing impressions of a broad spiral ,, ° 



° . r Maeropoma 



valve, are preserved in certain specimens in the Mantelli 

 British Museum. One species (M. Egertoni) is ( cll!lll 0- 

 from the Speeton clay ; the other (M. Mantelli) from the 

 chalk. 



Family XIII. — Sturionid^e. 

 The family Sturionidcc, represented by the sturgeons of 

 the present seas, makes its first appearance in the lias, under 

 the generic form of Chondrostcus, which has recently received 

 a full description and illustration in a memoir communicated 

 by Sir P. Egerton to the Royal Society of London.* In this 

 it is shown " that Chondrostcus, though essentially sturionian, 

 yet evidences a transitional form between the sturgeons and 

 more typical Ganoids ; that its food was similar to that of the 

 existing members of the family, but that it was procured in a 

 tranquil sea, rather than in the tumultuous waters frequented 

 by sturgeons at the present time." 



In the tertiary division of geological time the ganoid order 

 rapidly diminishes, and its place is taken by fishes with better 

 ossified internal skeletons, and with thinner, more flexible, and 

 usually soluble scales. The gills are supported on bony arches, 

 and are protected by branchiostegal rays, and by an operculum 

 or gill-cover. The aortic bulb is provided with but two valves ; 

 and the optic nerves decussate. For this group, including the 

 majority of existing fishes, and of those which made their 

 appearance during the tertiary period, Muller proposed the 

 name " Teleostei," which almost corresponds with the " osseous 

 fishes " of Cuvier. 



* Proceedings of the Royal Society, April 20, 1858. 

 I, 



