458 CALIFORNIA ACADEMY OF SCIENCES. 



ment. It appears as ;i simple Hap lying in front of, and, 

 when at rest, partly over the mandibles. 



The mandibles are large and strong, usually roughly 

 triangular in profile, with at least one projecting sharp- 

 pointed tooth, and one or two blunter teeth. Each man- 

 dible presents a characteristic facet and a condyle which 

 articulate with two strong chitinous rami attached one to 

 the ventral wall of the head and one to the dorsal wall. 

 One ramus articulates by a condyle with the facet of the 

 mandible, and the other by a facet with the condyle of 

 the mandible. The mandibular muscles are exceptionally 

 large. In the two genera of the suborder Amblycera, the 

 mandibles lie in a plane parallel to the horizontal plane of 

 the head, while in the two genera of the suborder Isch- 

 nocera, the mandibles project in a plane nearly or quite 

 at right angles with the horizontal plane of the head. 

 This remarkable difference is probably characteristic of 

 the two main groups of the order. The mandibles, though 

 varying somewhat in shape in the two groups, are essen- 

 tially similar in general character and in manner of artic- 

 ulation; the articulations in the Amblycera lie in a dorso- 

 ventral line, while in the Ischnocera they lie in a cephalo- 

 caudal line ; if, however, the Amblycerous mandibles be 

 assumed to be rotated through 90 , so that the anterior 

 aspect becomes directed ventrally, all of the apparent dif- 

 ferences in position of features and manner of articula- 

 tion between the mandibles of the two suborders become 

 reconciled. 



The maxilla are greatly reduced, the basal and termi- 

 nal sclerites so fused as to make it almost impossible to 

 differentiate them, and the structure so feebly chitinized 

 as to appear usually as a soft, small membranous lobe, 

 lying almost wholly concealed within the mouth cavity. 

 The inner border is sometimes chitinized, especially dis 



