sub-class i SELACHII 45 



resembles that of the sharks, while the dentition is distinguished by the remark- 

 ably peculiar form and structure of the few very large teeth or dental plates. 

 Of the four surviving genera, Chimaera lives off' the European coasts, Japan, 

 and the Cape of Good Hope ; Callorhynchus in the seas of the southern hemi- 

 sphere ; Harriotta in the deep Atlantic off North America ; and Ehinochimaera in 

 deep water off Japan. In all these genera the anterior dorsal fin is provided 

 in front with a strong spine, which is supported by a broad plate of cartilage 

 fixed to the vertebral column, and is united with this plate by a cartilage 

 articulation. Besides the fin spine, the males of recent forms bear on the 

 frontal region a spine which is rounded in front and covered with prickles, as 

 well as smaller prickly spines in front of the pelvic fins. The long copulatory 

 organs are also strengthened by thin calcified rods. The skin of the existing 

 genera is naked, but in some extinct genera it is partly covered with shagreen 

 granules and plates. A remarkable peculiarity of the Chimaeroids consists in 

 the unusually conspicuous character of the mucous-canals, which traverse the 

 skin of the head and form the very prominent lateral line of the trunk. These 

 are surrounded by numerous, closely arranged, calcified rings of cartilage. 



The existing representatives of the Holocephali are only an insignificant 

 remnant of a former much more extensively developed group of Selachians, 

 which appears first in the Devonian, and is perhaps genetically connected with 

 the Cochliodontidae. 



Family 1. Ptyctodontidae. Smith Woodward. 1 



Known only by dental plates, which are laterally compressed, seem to form a 

 single pair meeting at the symphysis of each jaw, and exhibit either feiu tritoral areas, 

 or sharp catting edges. Devonian. 



Ptyctodus, Pander (Aulacosteus, Eichwald ; Rinodvs, Newb. and Worth.). 

 Tritoral area composed of very hard, punctate, superimposed laminae, arranged 

 obliquely to the functional surface. P. obliquus, Pander, from Middle Devonian, 

 North Russia. P. molaris, East. ; Eifel Devonian. P. calceolus, Newb., and 

 P. ferox, East. ; Middle and Upper Devonian, Wisconsin, Illinois, and Iowa. 



Bhynchodus, Newb. Dental plates with sharp cutting edges, and no well- 

 differentiated tritors. North America, and the Eifel, Germany. 



Palaeomylus, Sm. Woodw. Robust teeth, with broad symphysial surface, 

 and one punctate tritoral area, not laminated. North America. 



Family 2. Squaloraiidae. Smith Woodward. 



Trunk depressed and elongated, and head produced into a long, slender snout. 

 Lower dentition comprising one pair, upper dentition two pairs of thin corrugated 

 dental plates without definite tritors. Males with a prehensile spine on the snout. 

 Dorsal Jin spine absent. Numerous conical or stellate dermal tubercles. Lower 

 Jurassic. 



Squaloraja, Riley (Spinacorhiniis, Ag.). Tail tapering to a point. Rostral 

 spine with expanded base, tapering to a point in front. Vertebral rings well 

 calcified, consisting of several concentric lamellae. S. polyspondyla, Ag., known 

 by nearly complete skeletons 0*45 in length, from the Lower Lias of Lyme 

 Regis, Dorsetshire. 



1 Eastman, C. R., Dentition of Devonian Ptyctodontidae (Amer. Xat. vol. XXXII.), 1898. 



