312 THE AMERICAN MONTHLY [Dec 
1), we shall see at once that the latter is only a portion 
of the carapace of the mother. The particular part of 
the shell which has been utilized for the purpose evi- 
dently consists of the valves (as distinguished from the 
head-shield), with the exception of a rather large piece 
of their ventral margins. The ventral margins have not 
wholly disappeared, for the characteristic shell-spines at 
the posterior ventral angle are still present. At first 
sight it does not seem that the portion of the shell now 
enclosing the resting egg has been specially modified. 
The ordinary faint hexagonal markings on the surface of | 
the valves are quite apparent, and the valves themselves 
are as transparent as when forming part of the coat of 
the living animal, ‘Towards tbe back there is, it is true, 
a somewhat darker tinge than usual, but this is not very 
noticeable, and taken by itself would scarcely suggést 
special modification. Looking more closely at the struc-- 
ture, however, it will be seen that at the back—i.e., along © 
the line representing the dorsal margin of the original 
valves, there is a distinct increase in the thickness of the 
chitin, and, further, that there is a narrow, highly re- 
‘fractive band of chitin. running somewhat obliquely 
across each valve from near the anterior dorsal angle to 
within a short distance of the posterior margin. It is 
the possession of these lateral thickened bands of chitin. 
which distinguishes the ephippium of Bosmina from all 
homologous structures. 
If one of the lateral bands of shila be still further ex- 
amined, it will be seen that it probably represents a modi- 
fied line of hexagonal areas, forming part of the shell 
sculpture of. the unaltered valve.. But all the cross par- 
titions have been obliterated, and the edges of the band 
have been smoothed so as to show but little indication of 
the original polygonal arrangement. The extremely 
minute granulation or pitting of the areas enclosed. by. 
the shell markings has also disappeared, for the chitin 
