LABIAL CARTILAGES OF BAIA CLAVATA. 
105 
tritt manchmal lateral iiber das Velum hinaus in den Boden 
der Nasenrinne, die er dann noch lateral mit begranzen hilft. 
Dem Nasenvelum gehort somit streng genommen nur der eine 
vordere, obere Lippenknorpel an.’ 5 
These several statements of Gegenbaur’s certainly 
definitely affirm that both the anterior and the posterior 
labials of either side of Raia enter into some part of the 
corresponding nasal flap, and they are apparently both 
said to extend into the tip of the flap. This is, however, 
evidently impossible, in so far as the so-called posterior 
labial is concerned, for in both Raia clavata and Raia 
radiata, which cannot differ markedly in this respect 
from Raia (species not given) and Raia vomer, the 
nasal-flap furrow lies definitely between the lateral portions 
of the two so-called labials, and it would necessarily 
continue so to lie however much the furrow might be 
reduced, or be extended inesially. The mesial edge, or 
bottom, of the furrow marks, or rather determines, the 
base of the corresponding* nasal flap, and the so-called 
posterior labial could not possibly enter any part of that 
flap, as the flap is found in my specimens, nor could it 
enter into a velum formed by the fusion, in the median 
line, of two such flaps. Gegenbaur’s several statements, 
above referred to, are accordingly certainly incorrect. 
The nasal flap of Raia and the other non-electric rays is 
said by Gegenbaur to be derived from the much smaller and 
quite different nasal flap found in most of the Selachii, the 
Scylliidae being said to present several intermediate stages in 
the process of development. A nasal velum is said to be 
formed in Myliobatis, in certain others of the non-electric 
rays, and also in certain of the Selachii, by the fusion, in the 
median line, of the nasal flaps of opposite sides. In the 
electric rays the method of development (Genese) of the 
velum is said (1. c., p. 221), to be totally different from 
that in the non-electric rays, the inference accordingly being 
that the vela in these two groups of fishes are equivalent but 
not homologous structures. This will be further discussed 
