312 A MANUAL OF DENTAL ANATOMY. 



rudimentary teeth seem not to be rudiments of a homo- 

 dont dentition as might have been expected, but of a 

 heterodont dentition ; and it is suggestive of a resemblance 

 to such forms as Squalodon, an extinct cetacean, peculiar in 

 having heterodont teeth. At all events it seems to indicate 

 that the homodont dentition of Cetacea is a case of degrada- 

 tion from ancestral forms, a conclusion likewise pointed to 

 by the gradual suppression of milk dentitions (see p. 300). 



From the upper jaw of an adult whalebone whale there 

 hang down a series of plates of baleen, placed transversely to 

 the axis of the mouth, but not exactly at right angles to it. 

 The principal plates do not extend across the whole width of 

 the palate, but its median portion is occupied by subsidiary 

 smaller plates. The whalebone plates are frayed out at 

 their edges, so as to be fringed with stiff hairs, and their 

 fringed edges collectively form a concave roof to the mouth, 

 against which the large tongue fits, so as to sweep from the 

 fringes whatever they may have entangled. The whale in 

 feeding takes in enormous mouthfuls of water containing 

 small marine mollusca ; this is strained through the baleen 

 plates, which retain the Pteropods and other small crea- 

 tures, while the water is expelled. Then the tongue sweeps 

 the entangled food from the fringe of the baleen plates and 

 it is swallowed. Each plate consists of two dense but 

 rather brittle laminae, which enclose between them a tissue 

 composed of bodies analogous to coarse hairs. By the pro- 

 cess of wear the brittle containing laminae break away, 

 leaving projecting from the edge the more elastic central 

 tissue, in the form of stiff hairs. 



Each plate is developed from a vascular persistent pulp, 

 which sends out an immense number of exceedingly long- 

 thread-like processes, which penetrate far into the hard 

 substance of the plate. Each hair-like fibre has within its 

 base a vascular filament or papilla : in fact, each fibre is 

 nothing more than an accumulation of epidermic cells, con- 



