Phyhgeny of the Tdeostomi. 331 



the articulation of the operculum; sjmplectic not ossified. 

 Branchiostegals not attached to epihjal and ceratohyal. 

 Gular plates, if present, not specially enlarged. Clavicle 

 distinct from tjie cleithrum. Notochord persistent. Peri- 

 cardium * communicating with the coclom. 



Order 2. CroSSOPTERYGII. 



Median fins with the dermal rays often in greater number 

 than their endoskeletal supports, which are often in two 

 series. Caudal heterocercal or diphycercal. Pectorals 

 lobate, with metapterygium often segmented. Ventrals lobate 

 or notj with supports variously arranged. Hyostylic. Bian- 

 chiostegals replaced by a pair of large gular plates. Clavicle 

 distinct from the cleithrum. Vertebral column variously 

 developed. 



Order 3. D i P n e u S T i. 



Median fins with the dermal rays In greater number than 

 their endoskeletal supports, which are in two series. Caudal 

 heterocercal or diphycercal. Paired fins acutely lobate, 

 with endoskeletal supports arranged as a segmented axis with 

 or without lateral branches. Autostylic, the palato-quadrate 

 being fused with the cranium and the hyomandibular reduced 

 or absent. Sometimes a pair of large gular plates, but 

 branchiostegal rays never present. Clavicle not distinct fro.n 

 the cleithrum. Notochord persistent. 



Order 4. P L A C D E R M i. 



]\Iedian fins membranous, without dermal rays, consisting 

 of a single dorsal, supported by regular series of baseosts and 

 axonosts, and a heterocercal caudal. Pectoral fin, if func- 

 tional, represented by a jointed Arthropod-like limb, with 

 internal muscles and external dermal plates, sometimes 

 reduced to a fixed spine, or absent. Ventral fin, if present, 

 with a series of baseosts and a single large axonostal cartilage. 

 ? Autostylic. Notochord persistent. Usually a well-deve- 

 loped dermal armour. 



Order 5. Teleostei. 



Median fins with the dermal rays equal in number to their 

 endoskeletal supports, which are typically in one series, the 



* I have taken this character from Bashford Dean, ' Fishes Living and 

 Foesil,' p. 200, 



22* 



