230 Tne Irish Naturalist. December 
The characters emphasised by itahcs in the above 
diagnosis are of especial value in distinguishing this species 
from P. maritimus, Leach (Carpenter, 19 13, pp. 3-5.) 
To facilitate discrimination between the two species, 
structural figures of some of the more important corres- 
ponding parts in P. maritimus are reproduced here (see 
Plate 2, figs. 1-9 for P. brevistylis and Plate 3, figs. L-IX. 
for P. maritimus, with explanations). Comparison of these 
drawings will prove more effective than the collation of 
long descriptions. To facilitate comparison, similar parts 
in the two species are indicated by corresponding numbers, 
arable in the case of P. brevistylis and roman for P. mari- 
timus. 
The contrast between the acute mandibular apex (fig. 
IIL, a.) of P. maritimus and the blunt truncated apex 
in P. brevistylis (fig. 3A.) is remarkably constant. The 
maxillary palps, and the stylets of the ninth abdominal 
segment relatively shorter and thicker in P. brevistylis than 
the corresponding structures in P. maritimus (compare 
figs. 6, VL, and 9, IX., st) are perhaps the best superficial 
features by means of which the two species may be distin- 
guished, and are common to both sexes. The prominent 
rounded lobes of the sub-coxae of the eighth abdominal 
segment (fig. 8, sc) in the male of P. brevistylis are also a 
strongly distinctive feature. As these lobes are absent, 
in the true P. maritimus, Leach (fig. VIIL) they should not 
be used to distinguish Petrobius in a sub -generic sense, as 
has lately been done by Silvestri (191 1). 
Petrobius brevistylis, the new species now described, is 
distinct not only from the common British and Irish P. 
maritimus Leach, but from the Dutch insect which Oude- 
mans designated Machilis maritima and described in his 
well-known memoir (1886). This last-named species, for 
which the name P. Oudemansi would be appropriate has, 
according to his figures, the lacinia of the maxilla much 
longer than the galea, the maxillary palp with the penulti- 
mate segment twice as long as the apical, the male gonapo- 
physes not reaching the tips of the ninth abdominal sub- 
coxae, and the penis attaining only to half the length of 
the ninth abdominal stylets. P. Oudemansi agrees, how- 
