434 california academy of sciences. 



The Mouth-Parts of Ancistrona gigas. 



(Plate lx, figs. 6-12.) 



In this form the full number of mouth-parts is present, 

 and all are complete except the maxilla? , these lacking- 

 palpi and distinct sclerites. The labium is well developed 

 and much longer than wide. It extends along nearly the 

 whole length of the ventral surface of the head, the an- 

 terior border being situated far forward, leaving only the 

 tips of the mandibles visible. When the palpi are par- 

 allel to the plane of the head their extremities project a 

 little beyond the anterior lateral margin. 



The labium (plate lx, fig. 7) consists of three distinct 

 parts, the ligula, mentum and submentum together with 

 prominent palpi. The mentum is the largest sclerite 

 and bears the four-jointed palpi. It is somewhat wider 

 than long, and is incompletely divided into three lobes, 

 one placed medially and the others on each side of 

 this. The median portion has the anterior border 

 straight or a little concave, and articulates with the 

 ligula. Each lateral lobe is narrow and projects an- 

 teriorly beyond the median portion which is much 

 wider. Each bears distally a palpus. The latter is 

 composed of four short joints of about equal length, 

 but the second is rather thicker than the others. The 

 lateral outlines of the mentum are roundly convex. The 

 median lobe forms, in part, a concavity on the surface of 

 the labium, but its posterior part forms a wide elevation, 

 continues externally with the elevated lateral lobes, and 

 is sharply marked off from the posterior part of the sunken 

 portion. This elevated posterior border bears a pair of 

 large, strongly chitinized, backward-projecting structures, 

 each being a sort of fork, with two prongs articulated 

 at their bases (plate lx, fig. 7, fir.). One of the prongs in 

 each fork lies dorsal of and a little within the other. This 



