2 Proceedings of ihe Royal Irish Academy. 



The following attempt accurately to review the parts in question in 

 Periflaneta australasice may be of use as a starting-point to those possessing 

 the necessary material, and desirous of seeing "how far Hansen's views find 

 support amongst forms allied to Periplaneta, attention moreover being 

 directed to one or two points of especial interest which have hitherto escaped 

 comment in this well-known genus. 



The work has been carried out in the Zoological Laboratory of the Eoyal 

 College of Science for Ireland ; and I am indebted to Professor G-. H. 

 Carpenter for his guidance and advice during this undertaking, and to 

 Mr. F. W. Moore of the Ptoyal Botanic Gardens for supplying me with 

 abundant material. 



The Mandibles (Plate I.) 



The mandible articulates with the head by means of the " condyle " (c) 

 and the " ginglymus " (g). The former is a knob-like projection on the posterior 

 surface, proximal and external in position, which works in a socket afforded 

 by the epicranial plate ; the latter, a shallow groove, plays upon a ridge of 

 the clypeus, and is situated anteriorly, some distance from the outer border 

 of the mandible. The axis of revolution of the jaw is thus directed forwards, 

 inclining slightly towards the middle line. The inner border bears some 

 distal teeth or blades, and a proximal truncated process, the " pars 

 molaris" (w_2j), the right mandible bearing three distinct blades, the left 

 having five such. When the jaws close, the processes are said to interlock, 

 which is certainly true with respect to the two molar surfaces, but the blades 

 of the right mandible all come to lie behind and across those of the left, the 

 third, or most proximal, on the right, being supported by the two extra 

 processes on the left (fig. 2). During mastication, I imagine that the molar 

 surfaces may, by closing upon and supporting the more resisting food-stuffs, 

 enable the overlapping blades to cut with better effect; the slight inward 

 inclination of the axes of rotation of the jaws tending to the same end. 



Below the pars molaris there is a well-marked process (/«) projecting 

 freely inwards, doubtless a homologue of the lacinia mobilis recorded by 

 Hansen ('93) and others as occurring in certain Coleoptera. Though 

 apparently figured by Muhr ('77), he makes no comment upon it. Miall and 

 Denny ('86) speak of a flexible chitinous flap, in Blatta orientalis, extending 

 from the inner border of the mandible to the labrum. As certainly no such 

 flap exists, those authors evidently refer to the lacinia mobilis, though 

 mistaken as to its true nature. 



The abductor, or extensor muscle of the mandible [Ex), arises from the 

 upper portion of the side of the external head skeleton, and is inserted by a 



