perfect State of Prosopistoma punctifrons. 75 
first seements of the abdomen, a coalescence which has super- 
induced a diminution of the respiratory parts and a more com- 
plete localization of those organs. In fact, instead of having, 
like the generality of the Ephemerina, a pair of respiratory 
organs or tracheo-branchie upon the sides of each of the tirst 
seven segments of the abdomen, and even presenting them 
upon other regions of the body (Oligoneurta, Palingenia), it 
possesses only five pairs of tracheo-branchie ; and, further, the 
first two pairs, which are considerably modified, are of little 
service in the phenomenon of respiration, although by their 
movements they contribute to facilitate the physiological action 
of the following ones. 
If, quitting Prosopistoma for a time, we examine the number 
and arrangement of the respiratory organs of the two types 
previously cited, in which an actual concentration is observed, 
we find :— 
First of all in Cenis or in Tricorythus* (fig. 1) the tracheo- 
branchie, more or less modified, are reduced to six pairs. The 
first pair, which is completely isolated in relation to the others, 
consists of two fusiform bodies placed one on each side on the 
margins of the first segment ; its physiological function must 
be almost null. 
We then find five plates of different dimensions upon the 
sides of the second, third, fourth, fifth, and sixth segments 
of theabdoment. The first consists of two very large and very 
strong plates destined to protect the following ones, which 
they can entirely conceal; these plates, which are of a horny 
consistency, present the same coloration as the integuments of 
the body ; if they do not present digitations on their margins, 
each of them nevertheless possesses a small tracheo-branchial 
tuft at its point of insertion. 
The other four pairs of respiratory organs are formed by 
* It is especially by studying the nymphs of Tricorythus, for which I 
am indebted to the kindness of Dr. E. Joly, that I have been able to 
ascertain the arrangement of the respiratory organs, and to recognize the 
perfect similarity that exists in this stage between that genus and the 
genus Cems. 
+ I venture here to point out a small error which Mr. Eaton has left 
in his monograph of the Ephemerina (Trans. Ent. Soc. London, 1871). 
In giving the diagnosis of the genus Cenis (p.92) he says “. . . Seg- 
mentorum abdomuinis 1, 3, 4, 5, 6, 7 branchifera;” whereas it ought to be 
“ Segmentorum abdominis 1, 2,3, 4, 5, 6 branchifera.” It is to the poste- 
rior margin of the second segment that the respiratory plate is articulated, 
and not to the margin of the third; for the latter bears the following 
tracheo-branchia, and. so on to the sixth segment inclusive. The seventh 
segment of the abdomen has never shown me respiratory organs, as, indeed, 
might be expected, seeing that they could not be protected by the plate 
covering the four preceding pairs. ae 
