316 AXXALS XEW YORK ACADEMY OF ^CIEXCES 



The Actinopten'gii also introduced several other important improve- 

 ments in the accessory locomotive structures. In the exoskeleton they 

 avoided overspecialization of the outermost layers, a cul-de-sac into which 

 the elasmobranchs entered, and while at lirst developing in due propor- 

 tion the ganoine, cosmine, vasodentine and isopedine strata, they avoided 

 the errors of the typical ostracoderms and antiarchians and did not 

 cumber themselves with a massive carapace and plastron. 



In the median and paired fins they were fortunate in evolving a type 

 of rod-like scales, which by fusing end to end, gave rise to the dermal 

 rays; these soon became the most important part of all the fin-web and 

 entirely superseded the horny fin-rays of an earlier period. TTith the 

 advent of these superior fin-rays and with the concomitant strengthening 

 of the vertebral column by neural and haemal rods, and finally by ossified 

 centra, the caudal fin in the Actinopterygii became of predominant func- 

 tional importance and changed from the heterocercal to the homocercal 

 type, receiving the powerful thrusts of the myomeres, which were trans- 

 mitted to it along the reinforced vertebral column. The hsemal rods 

 below the caudal column served to link the tail to the backbone. At first 

 they were slender and numerous, but gradually were reduced in number 

 and expanded into the broad h^-pural bones, around which the stout 

 dermal rays were tightly clamped. 



In the later Actinopterygii the body frequently became elongate, the 

 paired dorsal and anal fins were reduced, the dermal rays became reduced, 

 and the once powerful caudal degenerated into the pointed gephyrocercal 

 type. The varied history of scales and dermal rays of the Actinopterygii 

 illustrates the comparative rapidity with which these structures change 

 or disappear. The forerunners of the group may well have had scales 

 like those of the most primitive crossopterygians and dipnoans ; but even 

 in the Palaeoniscidae the ganoine has become many-layered and the cos- 

 mine layer is modified. In later Actinopterygii the ganoine and cosmine 

 disappear, the scales sink beneath the skin, lose their osseous tissue and 

 become horny. 



The over-development of the lepidotrichs or dermal ra^'s may perhaps 

 be responsible for the failure of the Actinopterygii to attain the highest 

 development of the endoskeleton of the median and paired fins. In the 

 earliest form (Cheirolepis) as restored by Smith "Woodward the pelvic 

 fin had a very extended base with short rod-like basals and radials, the 

 pectoral fin had a shorter ba*e, which was, however, wider than that of 

 later types : the dermal rays were long, numerous and scale-like. With 

 the collapse of the archiptervgial theory there is no longer any reason 

 why this wide-based type of fin that occurs so near the beginning of the 



