VERTEBEATA 17 



tain to the Cyclise. It embraces the simple family of the 

 Paleospondylidse based on the genus Paleospondylus, 

 which has been, so far, only found in the Carboniferous beds 

 of Scotland. 



Subclass III.— MARSIPOBRANCHII. 



This subclass has two orders: 

 Branchial fissures communicating directly 

 with the pharynx ; nasal sac perforating 

 the palate; Hyperotreti. 



Branchial fissures communicating with a 

 common branchial passage which opens 

 into the pharynx; nasal sac not perforat- 

 ing palate; Hyperoarti. 



To the Hypeeotreti belong two families, the Myxinidse 

 and the Bdellostomidse; to the Hyperoarti one, the Petro- 

 myzontidse. 



No extinct species of Marsipobranchii are known. They 

 include only the recent lampreys and hags. 



Class II.— PISCES. 



This class is divided into four subclasses : 

 I. No maxillary arch. 



A. No dermal cranial ossifications nor opercular 

 bones; claspers present (in living forms). 

 A hyomandibular; palatopterygoid arch 



distinct; Elasmobranchii. 



No hyomandibular ; palatopterygoid arch 



fused with chondrocranium ; Holocephali. 



AA. Dermal cranial ossifications and opercular 

 bones present; no claspers. 

 Suspensorium fused with chondrocranium ; 



palatopterygoid arch free; Dipnoi. 



