574 CLASS xvn. 



and contain a very small quantity only of bone-earth. In the 

 whales (Balcence) there are horny laminae (the so-named baleen) 

 in the upper jaw which stand transversely behind one another; the 

 lower jaw has neither baleen nor teeth, but, when the mouth is 

 closed, surrounds the outside of the baleen. The largest laminae 

 lie at the outer margin ; their form is that of a trapezium of 

 which the outside is much longer than the inside. Next to these 

 large laminae, on their inside, lie many smaller, which are also whiter 

 and softer. In every lamina an external and an internal substance 

 may be distinguished; the external or cortical substance consists of 

 horny plates lying close upon each other; the included medullary 

 substance is formed of parallel descending tubes which, at the inferior 

 margin of the horny lamina, pass into bristly fibres. These tubes 

 do not extend so high towards the superior maxillary bone as the 

 plates of cortical substance. Thus a space or cavity is left at this 

 part, which receives the germ-membrane, a fold of thick vascular 

 skin on which the balleens rest. This fold forms filamentous 

 elongations which proceed in the tubular, internal substance of the 

 baleen, and are accompanied by vessels in the same direction. 

 Between the laminae of baleen is a white substance, which seems to 

 require only a slight change to be transformed into horny matter 1 . 

 In the dolphins the teeth are not formed in sockets but in the gum, 

 and the margins of the jaw are elevated to enclose the teeth 2 . All 

 these teeth are conical and nearly similar; the middle teeth only 

 seem to be somewhat larger than the anterior and the posterior. In 

 this respect therefore the dolphins resemble many fishes and lacer- 

 tine animals. In the rest of the mammals the teeth are formed in 

 the alveoli. 



The tooth has a germ or matrix (pulpa dentis), by which the 

 bony substance (dentine OWEN) is secreted, or which is changed 

 into that substance by ossification, and of which the residue 

 occupies a cavity in the tooth when formed. The crown is 

 formed first and afterwards the root. The crown is covered with 

 enamel, a very hard substance which consists of fibres directed 

 towards the axis of the tooth. In the covered teeth (denies db- 

 ducti), as those of man, the quadrumanous and carnivorous animals, 



1 Compare F. C. ROSENTHAL AWiandlungen der Konigl. Alcad. der Wissenschaften 

 Berlin, aus dem Jahre 1829, Berlin, 1832, s. 127 132, Tab. I, in. 



2 HUNTER Phil. Transact. 1787, p. 398 ; see also OWEN Odontograpliy, pp. 358, 359. 



