J. K. Thaeher — Median and Paired Finn. 



301 



from the row of ]>arallel rays is presented in woodcuts A, B, C and D. 



The kind of 'cliange invoked is 

 simply coneresence, with scmie 

 spreading of the cartilage. The 

 former of these processes is abun. 

 dantly shown in the' case of the 

 median tins, while something of the 

 latter process is seen in PI. LI, figs. 

 12, 18, 14, 15; PI. LIII, fig. 27; 

 PI. LVI, fig. 46 ; PI. LVII, fig. 49. 

 And it is noteworthy that here the 

 rays which jn'olong themselves prox- 

 imally are the orad ones, just as they 

 are in the ventral fins. As for the 

 concrescetice, this has been carried 

 much farther in the dorsal ^n^ oi ^Sq^la- 

 lusAmericanus,3ryliobatis and Raia 

 levis than it has in the shark ventrals. 

 It is barely possible that the definiteness and constancy of the concres- 

 cence in the latter may be in whole or in part determined by the 

 copulatory function of the last part of the fin in male Elasmobranchs. 

 While the derivation of the ventral fins is thus easy from a series of 

 parallel cartilages, we find much greater difficulty in the ease of some of 

 the median fins, in Raia levis, for example, which is, unless my own 

 preconceptions deceive me, a far better case of a biserial archiptery- 

 gium than any furnished in the paired fins, aside from Ceratodics. 



C 



Indeed I may state that the origin of this i)aper lay in an observation 

 of a fin of another species of Haia, not however well enough preserved 



