514 PISCES FISHES. 



circular cavity, the rounded margin of which is adapted to a circular 

 groove in the contracted part of the base ; the margin of the tooth 

 which immediately transmits the pressure to the bone is strength- 

 ened by an inwardly projecting convex ridge. The masonry of 

 this internal buttress, and of the dome itself, is composed of hollow 

 columns, every one of which is placed so as to transmit in the 

 due direction the superincumbent pressure. 



" In another case, in which long and powerful piercing and 

 lacerating teeth were evidently destined, from the strength of the 

 jaws, to master the death-struggles of a resisting prey, we find the 

 broad base of the tooth divided into a number of long and slender 

 processes, which are implanted like piles in the coarse osseous 

 substance of the jaw ; they diverge as they descend, and their 

 extremities bend and subdivide like the roots of a tree, and are 

 ultimately lost in the bony tissue. This mode of implantation, 

 which I have detected in a large extinct Sauroid fish (Rhizodus), 

 is, perhaps, the most complicated which has yet been observed in 

 the animal kingdom." 



For a full account of the growth and developement of the teeth 

 of fishes, we must refer the reader to the same source from which 

 we have extracted the preceding paragraphs ; nevertheless, the 

 following is a brief abstract of Professor Owen's views upon this 

 subject. 



In all fishes the first step in the formation of a tooth is the 

 production of a simple papilla from the surface either of the soft 

 external integument, as in the formation of the rostral teeth of the 

 Saw-fish (Pristis), or of the mucous membrane of the mouth, as in 

 the rest of the class. In these primitive papillae there can be very 

 early distinguished a cavity containing fluid, and a dense membrane 

 (membrana propria) surrounding the cavity, and itself covered by 

 the thin buccal mucous membrane, which gradually becomes more 

 and more attenuated as the papilla increases in size. The pulp- 

 substance, or contents of the membrana propria, remains for some 

 period in a fluid or semi-fluid condition ; granules are ultimately 

 developed in it, which at first float loosely, or in small aggregated 

 groups, in the sanguineo-serous contents of the pulp. These gra- 

 nules soon attach themselves to the inner surface of the membrana 

 propria^ if they be not originally developed from that surface. 

 The whole of the contents of the growing pulp becomes soon after 

 condensed by the numerous additional granules, which are rapidly 

 developed in it after it has become permeated by the capillary 



