1 38 PALEONTOLOGY 





Genus Chimera. — The premaxillary teeth, one in each 

 bone, are oblong, about twice as high as they are broad, and 

 terminate below in a transverse trenchant edge; they present, 

 exteriorly, vertical columns of alternately harder and softer 

 substance, occasioning a notched margin when worn by use ; 

 interiorly, they have oblique laminae which do not extend to 

 the margin. The maxillary dental plates, one in each bone, 

 are triangular, and present a broad surface to the lower jaw. 



Genus Ischiodus, Egerton. — Each upper maxillary has 

 four dental columns; the lower jaw is less produced and 

 deeper than in Edaphodus. Of this genus, /. Johnsoni is from 

 the lias of Dorsetshire ; /. Egertoni from the Kimmeridge of 

 Shotover ; and /. Townshendi, a magnificent species, from the 

 Portland stone. Two species (J. Agassizii and /. brevirostris) 

 are from the cretaceous beds; at which period the genus 

 appears to have perished. 



Genus Ganodus, Egerton. — Maxillary dental columns 

 oblique, placed far back, converging as they advance, and 

 sometimes blending into one mass at the triturating surface. 

 This genus is exclusively represented by species from the 

 oolitic slate of Stonesfield — e. g., 67. Bucklandi, G. Colei, G. 

 Owenii. 



Genus Edaphodus, Egerton (including Edaphodon and 

 Passalodon of Buckland). — Each upper maxillary has three 

 dental columns; the lower jaw is more produced, but less 

 deep, than in Ischiodus : the premaxillary dental mass consists 

 of five vertical and slightly bent series of oblique and curved 

 transverse plates ; the median and longest series being 

 strengthened by a supplementary dental column behind; it 

 represents the genus Passalodon of Buckland. The large 

 E. Sedgwickii is from the greensand near Cambridge ; the still 

 larger E. gigas from the chalk of Kent and Sussex. The 

 ichthyodorulite called Psitlacodus Mantelli by Agassiz may be 

 the dorsal spine of this species. Three species, including the 



