the British Machilidse. 11 



It may be distinguished from the other British genus of 

 Machilidae, Prcumachilis, Grassi, by the presence of two pairs 

 of exsertile vesicles on the second to the fifth abdominal 

 segments inclusive, Preemachilis having not more than one 

 pair of vesicles on any abdominal segment. 



Genus Petrobius, Leach (1809). 

 Petrohius vectensis, sp. n. (Pis. I. & II.) 



Body-length 8 mm. (male). Feelers (incomplete) at least 

 as long as the body; tail-processes (incomplete) also at least 

 as long as the body. Feelers, cerci, and tail-process white- 

 ringed. Paired ocelli dumbbell-shaped, less than a transverse 

 diameter apart (PI. I. fig. 2). Mandible with blunt apex 

 (PI. I. figs. 3 &4«). Maxilla with lacinia slightly shorter 

 than galea (PI. I. figs. 6 & 6 a). Palp having its six elongate 

 segments with proportional lengths 6 : 8 : 10 : 11 : 11 : 10. 

 In the eighth abdominal segment of the male the subcoxse 

 are produced into prominent rounded lobes (PI. II. fig. 1). 

 The ninth abdominal segment has the subcoxse produced into 

 small pointed processes (PI. II. fig. 2). • Gonapophyses not 

 reaching quite to tips of subcoxse. Penis projecting to about 

 three-fourths length of stylet; stylets with long, acute, apical 

 spines. In the eighth abdominal segment of female the 

 subcoxa? are produced into acute processes projecting to 

 about \ length of stylet. Ovipositor of female nearly as long 

 as the cerci, which are less than half the length of the 

 incomplete tail-process. 



Loc. (1) Shanklin, Isle of Wight ; collected by Mr. E. 

 Popple, Beikhamsted, May 1913. (2) St. Helen's, Isle of 

 Wight; collected by Mr. P. A. Buxton, Aug. 1912. 



This species agrees with P. brevistylis, Carpenter (1913), 

 in having the mandible with blunt truncated apex, and 

 contrasts with P. maritimus, Leach (1809), the mandible of 

 which is acute. It agrees in many of its characters with the 

 Dutch insect which Oudemans designated Machilis maritima 

 and described in his memoir (1886). Prof. G. H. Carpenter, 

 in a paper on the "Irish Species of Petrohius" (1913), 

 referred to the latter insect, pointed out that it is certainly 

 not the true maritimus of Leach, and suggested P. oudemansi 

 as an appropriate name for it. Oudemans, in his drawings, 

 figures the lacinia of the maxilla much longer than the 

 galea; but in P. vectensis the lacinia is slightly shorter than 

 the galea, agreeing with P. brevisti/Us, Carpenter, while 

 the penultimate segment of the maxillary palp is not twice 



