PLAGIOSTOMES. 23 



the other. A vertical section of any of the teeth of the lamprey- 

 displays one or two cones of reserve, between the tooth in use, 

 and the surface of the matrix ;(1) and the outermost is in general readily 

 displaced ; often, indeed, difficult to preserve in situ in the preparation. 



Thin transverse sections of the teeth of the lamprey, viewed 

 by a magnifying power of a quarter of an inch focus, (2) exhibit their 

 structure as composed of closely-aggregated parallel tubes, placed 

 perpendicular to the secreting surface, and having a diameter of 

 4oo;JIh of an inch. 



In chemical composition, the teeth of the Cyclostomes resemble 

 horn. 



CHAPTER III. 



TEETH OF PLAGIOSTOMES. 



SHARKS, OR SQUALOIDS. 



12. All the genera of true or fixed-gilled plagiostomes, save 

 Pristis, are characterised by numerous teeth, which are restricted in their 

 situation to the upper and lower jaws. Here, they are arranged in a 

 greater or smaller number of rows, which succeed each other from 

 behind forwards, and are attached only to the mucous and fibrous 

 membranes covering the maxillary cartilages. 



Before entering upon the consideration of the teeth, a few words 

 seem necessary respecting the analogies of these dentigerous car- 

 tilages or jaws. In the common skate, they are in the form of 

 simple arches, each arch consisting of a pair of cartilages joined 

 together by a ligamentous symphysis at one extremity, and sus- 

 pended by the opposite end through the medium of a common car- 

 tilaginous pedicle from the sides of the cranium. This pedicle is 

 obviously the homologue of that to which the lower jaw is suspended 

 in the higher oviparous vertebrates, and which includes more or fewer 



(1) PI. 2, fig. 6. 



(2) The instrument employed in these observations, and referred to throughout the present 

 work, is the compound achromatic microscope of Ross. 



