BY R. J. TILLYARD. 381 



was a characteristic of the original Zygopterid stock before the 

 dififerentiation of the Agrionidce from tlie Calopterygidce was 

 fully established. 



Secondly, as regards the labium. Unlike the labia of Calop- 

 teryx and Hetoirina, that of Diphlebiah not deeply cleft medially; 

 in fact it possesses a median lobe of the usual Agrionid form. 

 Apart from this, the labium of Diphlehia is remarkable {a) for 

 its great size; (6) for the broad shield-shaped mentum, recalling 

 that of the jEschnida; (c) for its flatness, which also recalls that 

 of the jEschiidce. There is, however, a considerable resemblance 

 in the form of the lateral lobes in Diphlebia and Caloptevyx; each 

 carrying, besides the movable hooks, three fixed hooks or teeth. 

 Hetcerina appears to possess /onr fixed teeth, but the dentition in 

 this genus really consists of two parts, the upper one being itself 

 tridentate. Both lateral and mental setae appear to be present in 

 Hetcerina and Calopteryx, though in the latter genus they are 

 nearly reduced to vanishing point. No setce are present in 

 Diphlebia. 



Thirdly, as regards the caudal gills. Here the triquetral 

 character is even more pronounced in Diphlebia than in Galop- 

 teryx; for even the median gill of Dir)hlebia is distinctly trique- 

 tral and not lamelliform, though this gill is distinctly flatter and 

 more leaf-like than the lateral ones. It is a pity that a full- 

 grown Dijyhlebia nymph with perfect gills cannot be obtained, 

 for it must be understood that those sketched in the central 

 figure of the plate are, at best, approximate only, being drawn, 

 without actual measurements, from a sketch of a four-months-old 

 larva. It will be seen that in the young larva the gills were 

 hairy all over, but that, at four months, the hairs are restricted 

 to the outer two-fifths. Hence it is quite possible that when the 

 final nymphal stage is reached they may be completely hairless. 

 I am unable to find hairs on the shrivelled remains of the gills 

 attached to the exuvi?e, but their condition is not such as to 

 warrant the deduction that they may not have been there 

 originally. 



In these large triquetral gills we have clearly a form handed 

 down from remote antiquity. If the presence of numerous 



