508 F. G. SINCLAIR. 
examination shows that the legs of the thoracic region are 
single pairs as in other Myriapods. 
The external appearance of the head of the two species 
that I have worked at differs considerably owing to the pre- 
sence in P. mexicanus of a peculiar sense-organ mentioned 
by Humbert and Saussure in their description, in which they 
mention their resemblance to visual organs. “ Les deux 
points stematiform leur sont specieux et quoique nous n’ayons 
pas réussis a en distinguer nettement la nature, ils nous 
semblent cependant former des organes visuels, car ils sont 
révétus d’une cornée tres distinct.” 
On a first examination one can hardly help coming to the 
opinion that the organ in question is visualin function. Seen 
from the outside (figs. 8, 16) one sees that the external chitin 
is thickened so as to form a projecting boss, and further that 
the chitin of which this boss is formed is clear and transparent, 
so that the whole has a white appearance. One can also 
make out that the pigment layer that lines the internal side 
of the hypodermis is thickened just below the boss, and its 
colour is much darker, so that it forms a sort of iris. It is 
not, however, an eye, for there is no division into separate 
eye spots, and the external surface of the boss is not smooth, 
but when viewed under magnification is beset with fine hairs. 
The internal features of this organ will be described later in 
connection with the brain. 
P. mexicaunus has two small light-coloured patches in the 
frontal region, one on either side of the middle line. These 
small patches correspond with two depressions on the frontal 
region of the brain, but I have been unable to discover their 
function. 
The mouth parts consist of the upper lip, the hypostoma, 
and the mandibles. The first two of these have been described 
by Silvestri for P. polydesmoides (Classis Diplopoda, 
Portici, 1903), and my species offer no considerable variation. 
With the mandibles, however, it is different, as in both the 
species I have examined the mandible shows a considerable 
divergence from that of P. polydesmoides, as figured by 
