CHARACTERS 



279 



restricted sense as comprising the Actinopterygii and three other 

 orders, Osteolepidoti, Coelacanthini, and Polypterini. All these 

 orders, except the last, were already represented in Devonian times. 

 Their exact relationship to 

 each other remains a problem 

 to be solved. 



The chief characters of the 

 Teleostorni maybe summarised 

 as follows : The paired series 

 of bones preponderate on the 

 cranial roof. The pineal fora- 

 men is often preserved. Teeth 

 are borne on marginal dermal 

 bones on the jaws ; the skull is 

 hyostylic, the hyomandibular 

 large ; there is a palatal ptery- 

 goid compound plate. The 

 lateral gular plates are usu- 

 ally present. The notochordal 

 sheaths remain intact, although 

 generally the notochord is 

 constricted, and perichordal 

 vertebrae developed. The 

 series of neural and haemal 

 spines remains little, or not at 

 all, affected by the subdivision 

 and concentration of the 

 median fins. The two halves 

 of -the endoskeletal pectoral 

 girdle remain separate and 

 tend to dwindle in importance, 

 being to a great extent re- 

 placed by dermal bones; the 

 pelvic girdle is in the form 

 of paired ventral bones, or 

 cartilages. The skeleton of the paired fins may show distinct 

 signs of being built on the ' archipterygial ' plan ; but the 

 axis is generally posterior, and much reduced, or altogether absent 

 (Teleostei). All the fins are provided with lepidotrichia and 

 marginal actinotrichia. Besides the cosmoid, ganoid, or teleostean 

 scales which clothe the body, there is frequently preserved a 

 general superficial covering of dermal denticles. 



Living Teleostomes have the branchial rays supporting the gill- 

 lamellae in a double series ; a brain in which the prosencephalon is 

 not differentiated ; a separate anus and urinogenital aperture ; and 



Fio. 248. 



Ventral view of the pelvic girdle and left pelvic 

 fin of Salmo salar, L. (Modified from Bruch.) o.r, 

 posterior radial or remains of basipterygium ; I, 

 lepidotrich ; p, pelvic bone ; r, radial. 



