THE ORDERS OF TELEOSTOMOUS FISHES 457 



of the hyomandibular has not yet segmented off to form a 

 symplectic, although the jaw suspension is methyostylic. 1 (8) 

 The mandible has usually several dentigerous splenials on its 

 inner side. (9) The large gular plates are bordered by small 

 anterior and numerous lateral gulars. (10) The scales are bony 

 with a heavy coating of ganoine, and apparently represent 

 clusters of shagreen-like denticles. (11) The ribs are both 

 hypaxonic (haemapophyses) and epaxonic (parapophyses) . 



There is much evidence that the Crossopterygii are nearly 

 related to the Dipnoi and Amphibia (pp. 444 ). And in the 

 other direction Dean calls them "osseous sharks," because: (1) 

 The scales seem to indicate derivation from clusters of shagreen 

 cusps. (2) Polypterus retains a spiracle, an optic chiasma, and 

 shark-like viscera including a spiral valve and a conus arteriosus. 

 (3) The lobate paired fins may be interpreted as having been 

 derived from the non-lobate form seen in Sharks and in the 

 pelvic fin of Eusthenopteron. 



The Crossopterygii parallel the Actinopteri in (i) the replace- 

 ment of cartilage by bone, both in the endo- and exoskeletons, 

 (2) the aggregation and fusion of shagreen tubercles into scales 

 and plates, (3) the development (in the Ccelacanths) of the swim- 

 bladder as a hydrostatic organ and its ossification, as in certain 

 catfishes, (4) the adaptive radiation of the body form from the 

 primitive fusiform type into short -bodied and long-bodied types 

 (even an eel-like form, Calamoickthys, being at last evolved), 

 (5) the modification of the heterocercal tail into the diphycercal 

 and gephyrocercal types, (6) the reduction in number of the der- 

 mal rays for closer correlation with the endoskeletal supports 

 and the development of mobile fins supported by strong dermal 

 rays, (7) the reduction and proximal withdrawal (especially in 

 Ccelacanths) of the cartilaginous elements of the paired fins 

 pari passu with the increase in size of the dermal fin rays. Nos. 

 4-7 enable the movements of the internal skeleton to be trans- 



1 1, e. with the metapterygoid and opercular bones assisting the hyo 

 mandibular in the support or bracing of the quadrate or mandible. See 

 Gregory, "The Relations of the Anterior Visceral Arches to the Chon- 

 drocranium," Biological Bulletin, Vol. VII, No. 1, June, 1904, pp. 55-69. 



