PISCES. ELASMOBRANCHII. 63 



The best known of these primitive Elasmobranchs are the 

 Pleuracanthidae. 



Suborder (2). PLEUROPTERYGII. 



This suborder was formed for the reception of Cladoselache, 

 an Elasmobranch found in the Lower Carboniferous of Ohio 1 . 



The exoskeleton is in the form of small, thickly-studded 

 dermal denticles. The vertebral centra are unossified, and 

 the tail is strongly heterocercal. There were certainly five, 

 perhaps seven gill slits, and the suspensorium is apparently 

 hyostylic. The paired fins are, according to the view which 

 derives them by concentration from continuous lateral folds, 

 the most primitive known (see p. 129) and claspers are 

 absent. 



Suborder (3). SELACHII. 



Cartilaginous or partially calcified biconcave vertebrae are 

 always well developed ; they constrict the notochord inter- 

 vertebrally. The neural and haemal arches and spines are 

 stout and intercalary cartilages (interdorsalia) are present. 

 The tail is heterocercal, but in some cases (Squatina) ap- 

 proaches the diphycercal condition. In most cases the suspen- 

 sorium is hyostylic, the jaws being attached to the cranium 

 by means of the hyomandibular, and the palato-pterygo- 

 quadrate bar not being fused to the cranium. There are 

 generally five pairs of branchial arches, and gill rays are 

 borne on the posterior surface of the hyoid arch, and on both 

 the anterior and posterior surfaces of the first four branchial 

 arches. The J^otidanidae differ from most Selachians in two 

 respects, first as regards the suspensorium, Meckel's cartilage 

 articulating directly with the palato-pterygo-quadrate bar, and 

 not being connected with the hyoid arch ; and secondly as 

 regards the number of branchial arches, six pairs occurring 

 in Hexanchus and seven in Heptanchus. 



The pectoral fins are without the segmented axis of the 

 1 See B. Dean, J. MorphoL vol. ix. pp. 87114, 1894, and Nat. Sci. 

 voL vin. p. 245, 1896. 



