43§ CALIFORNIA ACADEMY OF SCIENCES. 



otherwise very similar to it. There are the same two 

 teeth, but the posterior one is a little thicker than on the 

 left side. The concavity on the ventral side is not so ex- 

 tensive, and the projection from the inner posterior angle 

 is shorter and thicker. The articulator} 7 condyle is sim- 

 ilar in position, but is perhaps a little larger. The facet, 

 however, is relatively more external. In each mandible 

 there is a considerable extent beyond a line drawn from 

 the condyle to the facet, but this is much shorter than that 

 in the opposite direction; i. e., inwardly. Each mandible 

 is supported by two large chitinous processes (plate lx, 

 fig. 9, d. ch. r. and v. ch. r.J, arising from a common 

 thick base which is attached to both the ventral and the 

 dorsal wall of the head. One of these processes lies ven- 

 tral to the other, and, in the case of the right mandible, 

 a little internal to it. The ventral projection bears termi- 

 nally a socket and the dorsal one a condyle; these artic- 

 ulate respectively with the condyle and facet of the man- 

 dible. The mandible is thus supported on an axis above 

 which is all its weight; and further, the greater part of the 

 latter falls on the inner side of the axis when the mandi- 

 bles are in the resting position. To the posterior edge 

 of the mandible, near the inner angle but still some dis- 

 tance from it, is attached, movably, a slender chitinous 

 rod. This rod passes posteriorly outward and dorsally, 

 and is attached to the dorsal wall of the head by bundles 

 of muscle-fibers. To the outermost projecting angle of 

 the mandible are attached directly other but much smaller 

 muscles. These have their insertion also on the wall of 

 the head, back of the mandible. These various inequal- 

 ities noticed on the two sides of the mandibular axes make 

 the strength of the closure of the mandibles much greater 

 than that of the opening. 



There are several internal mouth structures rather cu- 



