208 ON A FIDDLER. 



of Batoids, says " that the Batoid type of fin has been derived 

 from a shorter Selachoid one by forward rotation and general 

 enlargement is sufficiently clear from known facts of development.'^ 

 It is in these facts, viz., that the pectoral fin of Batoids undergoes 

 a forward growth in the embryo and only secondarily fuses with 

 the cephalic integument, that these cases of non-adherence in the 

 young or adult find their explanation. This is fully borne out by 

 the examination of a uterine embryo of Urolojjhus testaceus, 3 cm. in 

 length, in the teaching collection of the Biological Department of 

 this University. In this embryo in which distinct external gills 

 are present and the cranial flexure is well marked, the broadly 

 expanded pectoral fins extend forward beyond the mouth as two 

 blunt processes separated by a cleft from the head, and are at 

 this stage comparable with the adult condition of the pectoral 

 fins in Rhina. 



From these facts of development we are led to regard the non- 

 adherence of the anterior portions of the pectoral fins in Trygono- 

 rhina and the incomplete adherence of the anterior ends of the 

 fins in the other recorded cases as retentions more or less complete 

 of an embryonic or ancestral condition — as reversions in fact, for 

 if there is any truth at all in the law of recapitulation there can be 

 little doubt but that the free condition of the anterior portion of 

 the pectoral fin of Batoids was the primitive one. It is interesting 

 in view of this to find this feature of non-adherence most marked 

 in the Rhinobatid genus Trygonorhina, the Rhinobatids being 

 in many points transitional between the Batoidei and Selachoidei. 



In conclusion I have to express my indebtedness to Mr. Robert 

 Grant for the photograph from which the accompanying drawing 

 was made. 



EXPLANATION OF PLATE. 



Dorsal aspect of an abnormal specimen of Trygonor-hina fasciata — 

 reduced about 2j times. 



