HOLOCEPHALI 117 



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

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

 Shotover ; and 1". Toivnshendi, a magnificent species, from the 

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

 are from the cretaceous beds ; at which period the genus 

 appears to have perished. 



Genus Ganodus, Egerton (including Ganodus and Psit- 

 tacodus of Agassiz). — This genus is exclusively represented by 

 species from the oolitic slate of Stonesfield — e. g., G. Buch- 

 landi, G. Cold, G. Owenii. 



Genus Edaphodus, Egerton (including Edwphodon 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 strength- 

 ened by a supplementary dental column behind : it represents 

 the genus Passalodon of Buckland. The large E. Sedgivickii 

 is from the greensand near Cambridge ; the still larger E. gigas 

 from the chalk of Kent and Sussex. The ichthyodorulite 

 called Psittacodus Mantelli by Agassiz may be the dorsal spine 

 of this species. Three species, including the E. Bucklandi, 

 are found in the eocene of Bagshot and Bracklesham ; and one 

 species {E. Jielveticus) is from the mollasse of Switzerland. 



Genus Elasmodus, Egerton. — Each upper maxillary has 

 three dental columns, but the dentine is confluent, "being 

 rolled round like a scroll on the substance of the bone, one 

 edge forming the margin of the tooth, the other buried deep 

 in its centre.* The premaxillary has a thin incurved scal- 

 priform tooth, rounded at the cutting edge, of a lamellate struc- 

 ture, with a columnar arrangement of the plates, which are 

 juxtaposed. This genus is exclusively represented by species 

 — e. g., E. Hunteri — from the London clay of Sheppy. 



* Egerton, PYoc. Geol. Soc, May 12, 18 17. 



