SUB-CLASS I 



SELACHII 



25 



of Northumberland and in the Permian of Texas (Didymodus). Isolated 

 teeth and spines are very abundant in the Carboniferous and Permian. 



Anodontacanthus, Davis. Dorsal spines without denticles. Carboniferous; 

 England and Scotland. ~~ — ~ 



Order 4. PLAGIOSTOMI. Dum^ril. Sharks and Rays.^ 



Endoskeleton more or less calcified, hut no membrane calcificafions. Pterygo- 

 quadrate arcade movably articulated with the cranium. Vertebral axis segmented. 

 Paired fins with two or three shorf basal cartilages ; pelvic fins of male with appended 

 claspers. 



According to the condition of their vertebral column, the Plagiostomi 

 have been divided by Hasse into the four sub-orders of Diplospondijli, Cyclo- 

 spondyli, Asferospondyli, and Tectospondyli. 



Sub-Order A. DIPLOSPONDYLI. Hasse.. 



Notochord imperfedly segmented by vertical partitions. Gill clefts six or seven 

 in number on each side. Teeth numerous, in several series. Upper Jurassic to 

 Recent. 



Family 1. Notidanidae. 



Body fusiform, with a single unarmed dorsal and anal fin, and a large hetero- 

 cercal caudal fin. Teeth with two or more sharply-pointed cusps. Upper Jurassic 

 to Recent. 



Notidanus, Cuv. (Hexanchus, Heptanchus, Müller and Henle), (Figs. 44, 45). 

 Principal teeth consisting 

 of a series of compressed 

 cusps fixed upon a long 

 base ; all the cusps in- 

 clined in one direction, 

 the anterior larger than 

 the others, with or with- 

 out small denticles at its 

 base in front. Anterior 

 teeth of the upper jaw 

 clustered, awl-shaped ; a 

 median symphysial series 

 in the lower jaw. Prin- 

 cipal teeth of the upper 

 jaw less laterally elon- 

 gated, with fewer cusps 

 than those of the lower 

 jaw. Range from Upper 

 Jurassic to present day. 

 N. muensteri, Ag., repre- 

 sented by a complete 

 skeleton from the Lithographie Stone (Lower Kimmeridgian) of Solenhofen, 



1 Bonaparte, C. X., Selachorum tabula analytica (Mein. Soc. Sei. Nat. Neufchatel), 1839.— 

 Davis, J. W., Oll tlie Fossil Fishes of the Carboniferous Liniestone Series of Great Britain (Trans. 

 Roy. Dublin Soc. ser. 2, vol. I.), 1883.— //asse, C, Das natürliche System der Elasmobranchier auf 



Fig. 44. 



Dentition of the Recent Indian Grey Shark, Notidanus (Heptanchus) 

 indicus. 



