480 PEOr. ST. GEOEGB MIVAET ON THE 



pterygium. No structure exists in them which can be certainly considered the relic of a 

 primitively biserial limb, while their genesis from primarily distinct radials, significantly 

 indicated by a comparison of the dorsal fin of Noticlanus with its ventral fin on the one 

 hand, and with the dorsal fins of such forms as Ghiloscyllium on the other, seems all but 

 demonstrated by the simple ventral fin oi Polyodon. 



Thus, while I agree with Professor Humplirey as to the ventral and pectoral fins 

 being skeletal developments in lateral body-folds, I believe him to be quite mistaken in 

 regarding them as divaricated portions of the fold which forms the anal fin (the ventral 

 fold). The paired fins are of difi'erent origin ; and the ventrals have only assumed their 

 approximately mid-ventral position through adaptive developmental changes. 



On the whole, I feel strongly persuaded that Vertebrate limbs have been formed as 

 follows : — 



(1) Two continuous lateral longitudinal folds were developed, similar to dorsal and 



ventral median longitudinal folds. 



(2) Separate narrow solid supports (radials), in longitudinal series, and with their 



long axes directed more or less outwards at right angles with the long axis of 

 the body, were developed in varying extents in all these four longitudinal folds. 



(3) The longitudinal folds became interrupted variously, but so as to form two 



prominences on each side, i. e. the primitive paired limbs. 



(4) Each anterior paired limb increased in size more rapidly than the posterior 



limb. 



(5) The bases of the cartilaginous supports coalesced as was needed, according to 



the respective practical needs of the difi'erent separate portions of the longi- 

 tudinal folds, i. e. the respective needs of the several fins. 



(6) Occasionally the dorsal radials coalesced (as in Notidanus &c.) and sought 



centripetally (Pristis &c.) adherence to the skeletal axis. 



(7) The radials of the hinder paired limb did so more constantly, and ultimately 



prolonged themselves inwards by mesiad growth from their coalesced base, 

 till the piscine pelvic structure arose, as, e. ^.,in Squatiiia. 



(8) The pectoral radials with increasing development also coalesced proximally, 



and thence prolonging themselves inwards to seek a jjoint d'appui, shot dorsad 

 and ventrad to obtain a firm support, and at the same time to avoid the 

 visceral cavity. Thus they came to abut dorsally against the axial skeleton, 

 and to meet ventrally together in the middle line below. 



(9) The lateral fins, as they were applied to support the body on the ground, 



became elongated, segmented, and naiTowed, so that probably the line of the 

 propterygium, or possibly that of the mesopterygium, became the cheiroptery- 

 gial axis. 



(10) The distal end of the incipient cheiropterygium either preserved and 

 enlarged preexisting cartilages or developed fresh ones to serve fresh needs. 



