732 



MR. C. TATE BEGAN ON 



[June 19, 



pectoral radials articulated to the pectoral arch are present, but 

 there is no fusion of their proximal segments ; the basipterygium 

 is short and may be followed by a series of basalia *. 



The Acanthodii comprise Paljeozoic Selachians apparently 

 related to the Pleuropterygii, but with a strong spine at the 

 anterior edge of each of the fins. As in Cladoselachus mixo- 

 pterygia do not seem to have been developed, a feature which 

 distinguishes these two orders from the other Selachians. The 

 Ichthyotomi differ from the Pleuropterygii in that the anterior 

 pelvic basalia on each side have fused to form a pelvis, Avhilst the 

 pectoral fins are of a highly speciahsed type, the basalia forming 

 the segmented axis of a biserial fin and the anterior radials 

 articulated to the pectoi-al arch having been lost. In the 

 Euselachii the pelvis is a single unpaired piece ; basipteiygia are 

 formed in both pectoral and pelvic fins, and in the former tlie 

 proximal segments of the anterior radials have united to form 

 two cartilages, the propterygium and mesopterygium. In the 

 Holocephali the paired fins bear a considerable resemblance 

 to those of the Euselachii, but the pelves remain separate, and 

 the piece formed by the fusion of the anterior pectoral radials 

 does not seem to be the homologue of either propterygium or 

 mesopterygium. 



The order Euselachii includes all the living Sharks and Rays 

 and may be divided into two very natural and sharply defined 

 suborders, which may be compared thus : — 



Pleurotremata. 

 Eyes with free margins. 



Gill-openings lateral, the last in 

 front of or above the base of 

 the pectoral fin. 



Anterior (propterygial) mai-gin 

 of pectoral fin free. 



Pectoral radials typically simple 

 and of few segments, xtsually 

 only the anterior ones reach- 

 ing the free edge of the fin.t 



Two halves of the pectoral arch 

 well separated above (text- 

 fig. 123, p. 754). 



* Some authors liave placed Cladodus with the Ichthyotomi, a view not 

 endorsed either by Cope or Bouleuger. The structure of the pectoral is very similar 

 to that of the pelvic fins of the Ichthyotomi or of Syhodiis, as described by 

 Campbell Brown. 



f See exception in definition of the Lamnids. 



Hypotremata. 



Superior margins of eyes not 

 free. 



Gill- openings ventral, all below 

 the base of the pectoral fin, 

 which extends forward above 

 them. 



Anterior margin of pectoral fin 

 joined to the side of the body 

 or head,* the elongate pro- 

 pterygium lying at the base 

 of the fin. 



Pectoral i^adials ■ numerous, 

 multisegmented, distally bi- 

 furcated, all reaching the 

 free edge of the fin. 



Suprascapular cai'tilages either 

 united to the vertebral column 

 or else above it. 



