Glare Island Survey — Apterygota. 32 3 



Beyond the district the species extends its range all around the Irish 

 coast, both on the east and west. It is far commoner than the other shore- 

 haunting Petrobius (which I hope shortly to describe as a new species) ; and 

 I have no doubt that it is the form figured by Leach (1817, pi. 145) and 

 Lubbock (1873, pis. liv, lxvi). Specimens from the eastern Scottish coast, in 

 the Irish National Museum collection, are undoubtedly co- specific with the 

 Clare Island and common Irish insect. 



A species of Petrobius which occurs on the Dutch coast has been well 

 described and figured by Oudemans (1886), who calls it Machilis maritima 

 Latreille. It is certainly not the Irish and British species now under con- 

 sideration, as the sub-coxa of the eighth abdominal segment in the male is 

 prolonged into a rounded lobe, which is not the case in the true maritvnvus 

 of Leach. This character has been emphasized in a recent paper by Silvestri 

 (1911), who, apparently accepting Oudemans' species as certainly identical 

 with Leach's, has unfortunately defined a sub-genus Petrobius, characterized 

 by the feature just mentioned. As Petrobius was established by Leach for 

 his P. maritimus, Silvestri's nomenclature will require modification ; but as 

 I am doubtful of the value of sub-genera in a group where our knowledge of 

 specific distinctions is still very incomplete, I make no suggestion on the 

 subject here. Until the Continental Petrobii have been critically examined, 

 the range of P. maritimus outside the British Islands must remain doubtful. 



The genus Petrobius, according to the recent diagnosis of Silvestri (1904), 

 is distinguished from Machilis (with which it agrees in the presence of two 

 pairs of exsertile vesicles on each abdominal segment from the second to the 

 fifth inclusive) by the absence of genital processes (gonapophyses or paramera) 

 on the eighth abdominal segment in the male. There can, I think, be no 

 doubt that Verhoeff's genus Halomachilis, lately established (1910, pp. 428-9) 

 for an Adriatic species, is identical with Petrobius. Verhoeff points out an 

 easily observed character common to both sexes — the absence of scales on 

 the feelers beyond the two basal segments. Further the apex of the mandible 

 is without definite teeth. 



Diagnosis of Petrobius maritimus sens ste. 



Length, 25 mm. Peelers longer than body. Paired ocelli of the ordinary 

 dumb-bell shape, a transverse diameter apart (Plate I, fig. 1, o). Median ocellus 

 cordate, the apex directed upwards (fig. 1, om). Mandible with apex simple 

 and slender, slightly sinuate on the inner edge (fig. 2) ; slight indications 

 of teeth may be present in the male (fig. 3). Maxilla with tip of lacinia 

 projecting beyond or as far as top of galea (figs. 4, 5) ; palp T 4 3 length of 

 feeler, six elongate segments of palp with the proportional lengths 6 : 6 : 6 "5 : 

 9:8:6. The three terminal segments long and slender, only the antepen- 



A2 



