382 GEORGE H. CARPENTER AND MABEL C. MACDOWELL. 
anterior region of the labrum, to the left of the middle line, 
occur two small stout teeth About halfway back, 
slightly to the right, another prominent median tooth (t 2 ) is 
seen. Two arm-like sclerites [a. scl.) curve down from the 
anterior margin of the labrum and rest against two little 
toothed lobes (Z), which though smaller in size, correspond in 
position to the two maxillula-lobes (figs. 20, 21, mxl.) on the 
tongue, against which they evidently work. Below the 
median tooth (Z. 2 ) there is another long triangular median 
sclerite (m. scl.) being three little bosses. The structure is 
strengthened by lateral sclerites unevenly arranged and 
irregular in form. 
Rivers (*91, pi. ii, fig. 2) has figured the labrum of the 
larva of Dascillus Davidsonii, in which also there is a 
median-tooth and paired sets of lateral teeth. Rivers (loc. 
cit., p. 94) draws attention to the adaptation of these teeth, 
for biting in conjunction with those on the maxillulae and 
hypopliarynx described below, and suggests the analogy 
between this arrangement and that of a vertebrate jaw ! 
Mandibles. (PI. 36, figs. 13-15). 
In correlation with the exceptionally large size of the head- 
skeleton and the root-feeding habits of the larva, the mandibles 
are strong and formidable, and controlled by powerful 
muscles. As described by Gahan they are “ pointed at the 
apex, each with two cutting teeth on the inner side, and a 
slender movably articulated tooth (prostheca), between 
which and the large molar tooth there is a broad depressed 
space lined with a very pale-coloured integument.” 
Besides the two cutting teeth the left mandible bears a 
smaller tooth (fig. 13*) on its ventral aspect, situated below the 
prostheca (ps.) and overhanging the depressed triangular 
space (sp.). In some of the specimens examined the tip of 
this tooth bears rudimentary processes similar to those occur- 
ring on the maxillary spines. 
The prostheca of the left mandible is longer and more 
