1913- CarpknThr. — The Irish Species of Pcf robins. 231 
ever, with P. hrcvistylis in having prominent lobes on the 
sub-coxae of the eighth abdominal sternum. Whether the 
shore-haunting French species mentioned by Latreille 
(1832, p. 178) under the name of Machilis maritirna is 
identical with any one of the three Petrobii discussed in 
this paper must remain, for the present, doubtful. 
A few notes on the structure of the jaws in Petrobius 
may bring this paper to an end. It has been already 
mentioned that the absence of apical teeth on the mandible 
serves as a generic character. In a female specimen of 
the penultimate stage — apparently that distinguished by 
Verhoeff (1910, pp. 397-9) as " maturus junior," the cuticle 
of the apex and molar region of the final instar can be 
clearly seen within the part now functional (fig. 3). The 
tip of this new mandibular apex has small but quite distinct 
teeth ; apparently they are worn down almost as soon as 
the mandible comes into use. It is noteworthy also that 
there is apparently no new cuticle formed within the basal 
region of the mandible at this late stage, and the last moult 
undergone by the insect — at a stage when all its structures 
including the reproductive organs are developed — is in- 
complete. There is doubtless good reason for a fresh 
formation of the mandibular apex and molar surface which 
are subject to constant wearing action. 
The maxillulae (fig. 4, ML, IV.) are exceptionally well 
developed in insects of this family ; each maxillula shows 
a distinct lacinia and galea (fig. 4, IV., /, g), whose apices 
are finely spinose and reticulated, and a vestigial palp 
(fig. 4, IV., p.). The base of the maxillula is attached 
by a strip of membranous cuticle to the base of the median 
hypopharynx or tongue (fig. 4 hy), which has a granulated 
surface beset at the edge with minute bristles. The apex 
of the tongue is quadrate, emarginate centrally ; its base 
is supported by a pair of strong chitinous feet (fig. 4, pe) 
which articulate with the tentorium of the head-capsule. 
A somewhat complex "head" at the extremity of the 
lacinia of the maxilla (see fig. 6a) is characteristic of the 
Machilidae. This structure has been figured by Verhoeff 
(1904) from Machilis polypoda and by Borner (1908) from 
an undetermined Japanese Machilid. In Petrobius hrcvi- 
stylis the arrangement of parts does not differ markedly 
