94 MESSHS. HANCOCK AND ATTHEY ON 



darkness of colour ; on this layer there is a thin external coat- 

 ing of enamel. 



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

 thickness, in which the reticulation becomes somewhat closer, 

 and which is characterized by numerous short elliptical bone- 

 cells, the radiating canaliculi of which are frequently obliterated, 

 but in well-preserved specimens they can be observed distinctly. 

 The net-work of this stratum is continuous with that which lies 

 immediately above it, but is at once distinguishable by its darker 

 colour, greater density, and the presence of radiating cells. The 

 substance forming the reticulation of the upper portion of the 

 plate is, on the contrary, devoid of bone-cells, and is pale and 

 transparent ; but it is coated with a thin layer of a darker mat- 

 ter, in which are numerous branched tubules. When the section 

 is made very thin these tubules, however, ail disappear, and 

 the substance is then, to all appearance, perfectly homogeneous. 

 These tubules are likewise very frequently invisible, even in com- 

 paratively thick sections, probably on account of the state of the 

 fossil ; or it may be that the canaliculi have all disappeared un- 

 der the influence 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 beneath 

 it is frequently entirely worn away ; and then the section pre- 

 sents a rugged margin. 



The microscopic structure of Ctenodus has been figured and 

 described by M. Agassiz, in his "Poissons Fossils" (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 calci- 

 feres" at the base of the plate as without ramifications, it is 

 evident he has been deceived, probably by the use of balsam ; 

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

 in the specimen he examined. He is also wrong in his assertion 

 that " la substance qui forme la surface exterieure de la dent est 

 parfaitement homogene, sans trace de structure quelconque." If 

 his sections had been made very thin, this substance would un- 

 doubtedly have appeared so. The examination of many speci- 

 mens is frequently necessary to correct errors of this nature. 



