On some fossil fish-teeth of the genus CKRATODtlSj/row Maledi, South 

 of Nagpur, by Thomas Oldham, l. l. d., f. k. s., f. g. s., and 

 Superintendent of Geological Swvey of India. 



The genus Ceratodus was originally established by Agassiz,* to receive 

 several varieties of fossil teeth, presenting remarkable peculiarities in 

 external form, while agreeing in their internal structure and texture 

 with other teeth belonging to the genera Psammodits, Cochliodus, 

 Strophodus, &c,, included in his great sub-division of Placoids. 



These teeth are all composed of two distinct layers ; the upper, being 

 of enamel-like texture, composed of closely set tubes, and presenting 

 externally, where worn, a finely punctulated surface ; the other, or 

 lower layer, of bony structure, composed of reticulated tissue, like 

 that of cartilaginous fishes in general, this reticulated tissue being 

 closely set where the bone was thin, and loose and open, where it was 

 thick ; the external surface, where perfect, being smooth. 



The form of these teeth was described as presenting on one side a 

 smoother edge or border, straight or approximately straight through a 

 portion of its length and curved in the remainder, while the opposite 

 edge or border of the tooth was marked by several projecting ridges or 

 horns ; whence the name of the genus. 



Under this genus, Agassiz separated and gave specific names to four- 

 teen different varieties, stating distinctly, however, that he did so only 

 provisionally, until the discovery of better preserved and more numerous 

 specimens enabled him to determine how far the differences he had no- 

 ticed depended really on any specific distinctness, how far on the position 

 of the teeth in the upper or lower jaws, or how far on age. And cer- 

 tainly, as has been forcibly pointed out by Quenstedt,| the specimens fi- 

 gured by him seem to demand this caution from their imperfect condition. 



* Agassiz. Rechcrches sur Ics Poissons fussiles. Vol. III. p. 129. 

 t Ilanilbiicb der Petrefactcnkimde, p. 186-7. 



