726 



MR. C. TATE REGAN ON 



[June 19, 



The relations of the principal groups ma.y be diagrammatically 

 expressed thus : — 



Nareobatoidei. Batoidei. 



(Hypotremata.) 



Galeoidei. 



Squaloidei. 

 Notidanoidei. 



(Euselachii Pleurotremata 



IchtliA^otomi. 



Holocephali. 



(Cliasmatopnea.) 



Acanthodii. 



Pleuropterysjii. 



"I 

 (Selachii Trematopnea.) 



The, diagnostic features of the two series may be compared as 

 follows : — 



Trematopnea. 

 Gill- clefts opening directly to 



the exterioi'. 

 Pterygo-quadrate distinct from 



the cranium. 



Cliasmatopnea. 

 Gill-clefts opening into a chamber 



with a single external aperture. 

 Pterygo-quadrate fused with the 



cranium. 



or infrahfemals are the ventral counterparts of the supraneurals is shown especially- 

 well in the Dipneusti, the ribs meeting in the caiidal region to form haemal spines, so 

 that the ventral arches are exactly similar to the dorsal ones. In the Chondrostei 

 infrahasmal elements are wanting in the posterior abdominal and caudal regions. In 

 ■fishes with a bony vertebral column the presence of centra and the co-ossification of 

 the neural and supraneural and in the caudal region of the haBUial and infrahjemal 

 elements usually obscure the structure. I would, tlien, define the Selachians as 

 having neural and htemal arches onlj^, and the Teleostomes as having not only neural 

 and hsemal arches, but also supraneural and infrahasmal arches. 



