402 MR. J. G. H. FEEW : MORPHOLOGY OF THE HEAD CAPSULE 
SO that these two points become coincident, the anterior arm now being 
represented only by the portion between the points and P. Although this 
explanation seems quite plausible^ an examination o£ Peterson's extensive 
series of figures has failed to reveal any morphological evidence in support 
of it in the shape of intermediate forms, and in view of the extensive series 
of forms studied by Peterson it seems unlikely that such evidence is available. 
Peterson (p. 16), although he says that the anterior arms of the epicranial 
suture are not present in any Dipteron possessing a ptilinum, considers, 
nevertheless, that " there is every reason to believe that the tentorial thick- 
enings mark the course of the suture." If one accepts this view the region 
of the head lying between the dorsal arms of the tentorial thickenings in 
Chlorops must be the f rons ; but owing to the curved dorsal ends of the 
thickenings the antennae arise, quite definitely, on this included region. 
Such a position for the antennaa does not occur, so far as I know, in any 
insect, and one must, therefore, conclude that the region lying between the 
dorsal arms of the tentorial thickening is not the frons (or frouto-clypeus). 
I consider that the tentorial thickenings between the points X and Y on 
each side mark the lower portions of the arms of the epicranial suture, the 
shallow invaginations in which the thickenings lie being probably the actual 
sutures which have remained permanently open. As mentioned above, the 
dorsal edge of the oral depressioa is formed by a transverse chitinous bar 
uniting the two halves of the tentorium, and I consider that this represents 
the dorsal and median region of the arms of the epicranial suture. The 
following passage from Peterson (p. 28) gives, I think, considerable support 
to this view. Speaking of the invaginations of the anterior arms of the 
tentorium in Tabanus he says : — " The invaginations on each lateral half of 
the head are joined together by the arms of the epicranial suture and 
resemble the hypothetical type." The points Y have already been indicated 
as probably representing the points of invagination of the anterior arms of 
the tentorium. 
I consider, therefore, that the dorsal and lateral margins of the oral 
depression coincide with the anterior arms of the epicranial suture. Accord- 
ing to this view the antennas arise on the vertex, a position which they 
occupy in many Orthorrhapha, and one must look for the frons and clypeus 
among the structures lying in the oral depression. On the dorsal surface of 
the rostrum of the proboscis there is a plate which is the superficial region 
of a chitinous structure usually termed the fulcrum. Lowne (p. 134) 
regarded this :is the clypeus, and Dr. Imms tells me that he is expressing the 
same view in a forthcoming publication. With this view I agree, with the 
small reservation that, as according to Peterson (p. 17) the frons and clypeus 
are separated by a doubtful suture in only a few Orthorrhapha* and are 
entirely fused in the majoritj^ it is possible that the sclerite in question 
* He also regards the frons and clypeus as fused in the Cyclorrhaphn,but here the region 
which he calls fronto-clypeus is, in my opinion, part of tlie vertex. 
