228 The hish Naturalist December 
THE IRISH SPECIES OF PETROBIUS. 
BY PROFESSOR GEORGE H. CARPENTER, B.SC, M.R.I.A. 
(plates 2, 3). 
(Read before the Dublin Naturalists' Field Club, 9th December, 1913). 
Bristle-tails or " Rock-jumpers " of the species described 
more than a century ago by Leach (1809) as Petrohius 
■maritimus and recorded and ligured in Lubbock's well- 
known "Monograph of the Collembola and Thysanura " 
(1873) as Machilis maritima, are very' common around the 
Irish and British coasts. They may usually be found 
under the stones of the beach, or in the cracks of rocks 
close to high-water mark, but they are by no means con- 
fined to the tidal margin. On Howth Head, for example, 
specimens inhabit the tops of the cliffs, and on Clare Island, 
Co. Mayo, these insects abound among the loose stones 
of the walls that border the roads along the shore. 
In a recent paper (19 13) contributed to the Natural 
History Survey of Clare Island, I have defined and figured 
with some detail the characters of the common Petrobius 
which may be regarded as P. maritimus Leach, and I have 
incidentally pointed out that a second and very distinct 
species of Petrobius is found in Ireland. This latter is 
much rarer than its relation, and so far, specimens have been 
obtained nowhere else than at Portraine, Co. Dublin. 
Genus Petrobius, Leach (1809). 
Petrobius, Silvestri (1904). 
Halomachilis, Verhoeff (1910). 
When Leach established Petrobius, he was not able to 
distinguish it from IMachilis by very definite characters ; 
the only outstanding feature in his diagnoses, indeed, is 
the 'labial palp of which, in Machilis, the apex is said to be 
membranaceous, wliile Petrobius has " the last joint 
obliquely truncate, with the apex acute and not mem- 
branaceous." It is not surprising therefore that for many 
years his species P. maritimus was relegated to a compre- 
hensive genus Machilis co -extensive indeed with the family 
Machilidae. In recent studies of the genera, Silvestri has 
now shown, however, that while Machilis (sensu str.) has 
