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. 



A nodont acanthus, Davis. Dorsal spines Avithout denticles. Carboniferous; 

 England and Scotland. 



Order 4. PLAGIOSTOMI. Dumeril. Sharks and Rays.^ 



Endoskeleton more or less calcified, but no membrane calcifications. Pterygo- 

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

 Paired fins with two or three short basal cartilages ; pelvic fins of male with cqjpended 

 claspers. 



According to the condition of their vertebral column, the Plagiostomi 

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

 spondyli, Asterospondyli, and Tectospondyli. 



Sub-Order A. DIPLOSPONDYLI. Hasse. 



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

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

 Recent. 



Family 1. Notidanidae. 



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

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

 to Eecent. 



Notidanus, Cuv. {Hexanchus, Heptanchus, Miiller 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, Avith or Avith- 

 out small denticles at its 

 base in front. Anterior 

 teeth of the upper jaAv 

 clustered, aAvl-shaped ; a 

 median symphysial series 

 in the loAver jaAv. Prin- 

 cipal teeth of the upper 

 jaAv less laterally elon- 

 gated, Avith fcAver cusps 

 than those of the loAver 

 jaAv. Range from Upper 

 Jurassic to present day. 

 X. muensteri, Ag., repre- 

 sented by a complete 

 skeleton from the Lithographic Stone (LoAver Kimmeridgian) of Solenhofen, 



^ Bonajxj/i'te, C. L., Selachorum tabula analytica (Mem. Soc. Sci. Nat. Neufchatel), 1839. — 

 Davis, ./. IF., On the Fossil Fishes of the Carbouiferons Limestone Series of Great Britain (Trans. 

 Roy. Dublin Soc. ser. 2, vol. I.), 1883. — Hasse, C, Das nattirliche System der Elasmobranchier auf 



Fig. 44. 



Dentition of the Recent Indian Grey Shark, Kotidanus (Heiitanchus) 

 indicus. 



