Lampreys and Sharks. 23 



one median pitlike nostril, and the hag is remarkable 

 among fishes for having a passage of communication 

 between the bottom of this pit and the posterior part 

 of the cavity of the mouth. No such communication 

 exists in other fishes, in which the nose is a simple 

 depression or cavity on the surface of the head, lined 

 by a plaited mucous membrane and crossed over by 

 a. bridge of skin. The teeth in lampreys are horny 

 and conical ; they are shown in fig. n. 



FIG. IT. 





Head of lamprey dissected. 



, I, c, cartilages of the mouth ; d, e,f,g, h, muscles attached 

 to the cartilages. 



The name Marsipobranchii is given to this group 

 on account of the pouchlike nature of the gills, which 

 are in six or seven pairs, arranged in two lateral rows, 

 and open by small holes on the surface. 



15. Order, 2, Selachia (Sharks). The second 

 order includes the sharks and rays, the largest indi- 

 viduals in the entire class of fishes ; but, notwith- 

 standing their size, these exhibit in respect to many 

 points of organisation what may be considered as 



