And MOUTH-PAKTS of CHLOEOfS T^NIOPUS. 
401 
It seems clear that the bars extending from the points Y to the antennal 
bases are the dorsal arms of the tentorium, and their antennal ends ai'e 
probabiy to be regarded as their points of invagination. The bars on each 
side uniting the points X and Y are here regarded as the anterior arms of 
the tentorium, their dorsal ends (Y) being their true points of invagination. 
As already mentioned, their invaginations are actually slit-like and extend 
along their whole length. Peterson has described several Brachycei-a and 
Cyclorrbapha Ascliiza in which similar, though apparently not such pro- 
nounced, slit-like invaginations run along the anterior arms of the epici-auial 
suture, and the occurrence of slit-like invaginations in Chlorops cannot, 
therefore, be regarded as in any way exceptional. It thus seems clear that 
the three arms of the tentorium can be recognized in Chlorops as in less 
Fig. 3. 
Diagram illustrating the nature of the tentoiial thickenings of Chlorops. 
specialized insects, though their relations to each other are undonbtedly very 
different from the relations in more typical tentoria. Peterson, speaking of 
generalized insects, says (p. 26) : " The small dorsal arms unite with the larger 
anterior arms, and these, in turn, join with the posterior arms." Fig. 3 A 
represents this diagrammatically ; the points and P represent respectively 
the points of junction of the dorsal arms with the anterior arms, and of the 
latter with the posterior arms. Fig. 3 B represents diagrammatically the 
condition found in Clilorops, and the following provisional explanation is 
offered. In Cliloroios, as in most species possessing a ptilinum, the rods of 
the tentorium have become fused with the head capsule. In the case of the 
anterior arm this may be supposed to have resulted in the entire obliteration 
of the portion of the rod extending from a.i. (its point of invagination) to 0, 
