308 
to the small size of my specimen, I found the extrabranchials very 
undeveloped, extending only to the upper surface of the gill plates 
and not curving over them. 
Taking up the other orders of Elasmobranchs, in Scyllium 
canicula I found, with Wurre, three ventral and four dorsal extra- 
branchials, but in Squalus acanthias I found five on either side. 
In both skates and sharks these cartilages show a definite and 
characteristic form and structure peculiar to their own orders. In 
the sharks they show no signs of fusion, and are well-chondrified, 
rod-like structures. In the skate, although the form and size differ 
much in different species, great flexibility and thin, plate-like struc- 
ture are generally characteristic. 
Chimaera representing the third remaining order of Elasmo- 
branchs, shows no traces of the extrabranchials. This is significant 
in connection with the fact that the five gill openings are reduced to 
one, and the gills are covered by an opercular fold. 
None of the morphologists who mention the extrabranchials offer 
any explanation of them. They are apparently homologous 
with the branchiostegal rays, either by fusion of the 
extremities of the rays, or by a modification of the 
first ray, or possibly by both processes. The evidences 
for such a homology are two: first, the similarity in form and struc- 
ture of the branchiostegal rays and extrabranchials in both skates 
and sharks, respectively, both structures of the former being broad 
and flexible, of the latter rod-like and more chondrified; secondly the 
actual cases of fusion that I have found in Raja radiata and 
Torpedo ocellata. 
In the sharks these structures seem to have become more spe- 
cialized. If they were originally formed by fusion, they have lost all 
traces of the process and have become independent; if they originated 
as branchiostegal rays, they have changed their position and have 
become enlarged and modified. In the different species of the skates, 
they are very variable and show, as in the cases cited, intermediate 
stages of development. 
The function of the extrabranchials seems to be 
that of protection to the underlying gills. So far as I have 
found, they exist in all those fishes that possess a number of gill 
openings and consequently no opercular fold. 
Northampton, Mass. U. S. A., 
January 11, 1897. 
