ELASMOBRANCHII ICHTHYOTOMI 



439 



is acentrous, and the persistent notochord 

 basidorsal cartilages, which alter- 

 nate with small interdorsals, a 

 series of basiventrals supporting 

 small ribs, and in the caudal 

 region well - developed haemal 

 arches. The dorsal fin is sup- 

 ported by slender, tri-segmented 

 radialia, which appear to be 

 twice as numerous as the neural 

 arches in the trunk, but in the 

 dorsal lobe of the caudal fin 

 the two structures agree in 

 number. Ventrally - prolonged 

 haemal spines are the sole endo- 

 skeletal supports of the inferior 

 lobe of the caudal. The coraco- 

 scapular cartilages of opposite 

 sides remain distinct, and each 

 supports a biserial fin. The 

 pelvic girdle is represented by 

 a pair of small cartilages sup- 

 porting the basipterygia. The 

 pelvic fins are uniserial, with 

 post-axial skeletal supports for 

 claspers in the males. Both 

 the median and the paired fins 

 are provided with marginal 

 ceratotrichia. The skull is 

 probably amphistylic. Five, 

 possibly six or seven, branchial 

 arches, bearing clusters of 

 minute denticles, are present. 

 Circumorbital plates are want- 

 ing. All the endoskeletal struc- 

 tures are partially calcified. The 

 teeth are tricuspid, each with 

 two long divergent lateral cusps 

 and a minute median cusp ; the broad bases 

 and articulate with one another by means of 



supports a series of 



of the teeth overlap 

 facets. 



