356 Messrs. Hancock & Atthey on BejHile- and Fish-Remains 

 account of the microscopical characters of these peculiar dental 



In sections made across the transverse ridges that cover the 

 whole surface of the plates, a very beautiful structure is pre- 

 sented to view. The entire substance is found to be com- 

 posed of a minute reticulation of bone-like matter, the meshes 

 or medullary canals being large and much complicated. The 

 ridges stand up from the surface in the form of conical tooth- 

 like processes ; and the reticulated matter of which they are 

 composed is perfectly continuous with that of the plate or 

 base ; but the meshes or medullary canals in them are a little 

 elongated, and the surface is protected by a compact, rather thin 

 layer, which is only distinguishable from the rest of the tissue 

 by its density and darkness of colour ; on this layer there is a 

 thin external coating of enamel. 



At the base of the plate there is a stratum of considerable 

 tliickness in which the reticulation becomes somewhat closer, 

 and which is characterized by numerous short elliptical bone- 

 cells, the radiating canaliculi of which are frequently oblite- 

 rated, but in well-preserved specimens they can be observed 

 distinctly. The network of this stratum is continuous with 

 that which lies immediately above it, but is at once distin- 

 guishable by its darker colour, greater density, and the pre- 

 sence of radiating cells. The substance forming the reticula- 

 tion of the upper portion of the plate is, on the contrary, de- 

 void of bone-cells, and is pale and transparent ; but it is coated 

 with a thin layer of a darker matter, in which are numerous 

 branched tubules. When the section is made very thin, these 

 tubules, however, all disappear, and the substance is then to 

 all appearance perfectly homogeneous. These tubules are 

 likewise very frequently invisible, even in comparatively thick 

 sections, probably on account of the state of the fossil ; or it 

 may be that the canaliculi have all disappeared under the in- 

 fluence of the balsam used in mounting the specimens. 



The peripheral enamel is very often wanting ; and even the 

 dense continuous layer of bone-like matter immediately be- 

 neath it is frequently entirely worn away; and then the section 

 presents a rugged margin. 



The microscopic structure of Ctenodus has been figured and 

 described by M. Agassiz, in his ' Poissons Fossiles ' (vol. iii. 

 p. 166, tab. M. f. 3). The figure is very good, so far as it is 

 worked out ; but when the author describes the " cellules 

 calciferes" at the base of the plate as without ramifications, it 

 is evident he has been deceived, probably by the use of bal- 

 sam ; or it is just as likely that the canaliculi had not been 

 preserved in the specimen he examined. He is also wrong in 



