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 Urolophus 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 Trygonorhina fasciata — 
reduced about 2\ times. 
