330 Mr. C. T. Regan on the 



development of membrane-bones, including an operculum * 

 covering the chamber into which the gill-clefts open. 



The Teleostomi may be divided into five orders, the rela- 

 tions of which are expressed in the following diagram : — 



Teleostei. 



Dipneusti. Plapodernii. 



/ .. 

 Crossopterygii. 



Chondrostei. 



The Chondrostei and Crossopterygii correspond to the 

 groups usually so named ; the Dipneusti comprise the 

 Sirenoidei only ; the Placodermi include the Arthrodira, 

 Antiarcha, and' Osteostraci ; and to the Teleostei the Ganoidei 

 Holostei are added. 



These orders may be defined as follows : — 



Order 1. CHONDROSTEI. 



Median fins with the dermal rays in greater number than 

 their endoskeletal supports, which are typically in two prin- 

 cipal series, baseosts and axonosts, with an outer series of 

 small marginal cartilages. Caudal typically completely 

 heterocercal (rarely abbreviate heterocercal or diphycercal). 

 Paired fins not notably lobate. Pectoral baseosts articulating 

 with an anterior coraco-scapular cartilage and a posterior 

 metapterygium f. Ventrals with a well-developed series of 

 baseosts articulating internally with a series of axonosts, 

 which may be separate or more or less completely fusefl. 

 Hyostylic. Hyomandibular without posterior process for 



* In some specialized forms (e. g. Aspredinidge) the operculum is 

 ■wanting. 



t It is impossible to say whether in the most primitive Teleostomi the 

 metapterygium was already developed or whether it was represented by 

 a series of separate axonosts. 



