938 



CLASS PISCES. 



there is but one dorsal fin, and the number of the gill-clefts exceeds 

 five. 



Family Notidanid^. — The single family of this series is repre- 

 sented by the genera Notidanus and Chlamydoselache, both of which 

 inhabit the warmer seas. The dorsal fin has no spine, and the teeth, 

 of which several series are in use at the same time, have sharply 

 pointed cusps. Notidanus (Heptanchus, Hexanchus) is readily 



Jf* Orl Ff^CTK 



Fig. 861. — Left lateral view of the skull of Notidanus. Recent. Reduced. R, Rostrum ; 

 AF, PF, Pre- and postorbital processes; Orb, Orbit; NK, Nasal capsule; +, Articulation of 

 palatopterygoid (PQ) ; G, Articulation of Meckel's cartilage (Md) ; Z, Teeth ; IVS, Vertebral 

 column. (After Wiedersheim.) 



characterised by the inferior position of the mouth and the peculiar 

 form of the lateral teeth. These lateral teeth (figs. 86 1, 862) are 

 comb-like, consisting of a series of compressed cones, inclined in 

 one direction, and fixed upon an elongated base ; the anterior cone 

 being the largest, and frequently having cusps at the base of its 



anterior border. The lower teeth 

 are more complex than the upper, 

 and the anterior teeth in both jaws 

 are simple and awl -shaped. The 

 primitive position of this genus is 

 shown by the persistent notochord 

 and the characters of the skull 

 (fig. 861). The latter approaches 

 the so-called amphistylic type, differ- 

 ing from that of all other existing 

 Selachians by the slendemess of the 

 upper part of the hyoid arch, which 

 does not support the palatopterygoid 

 bar and mandible. The palatoptery- 

 goid is connected, however, with the cranium by a distinct facet 

 articulating with the postorbital process. This structure is re- 



Fig. 862. — Lower teeth of Notidanus 

 gigas ; from the Red Crag. The an- 

 terior cusps are worn away in the lower 

 figure. (After Smith-Woodward.) 



