120 MR. R. I. POCOCK ON THE SCORPIONS [^lar. 18, 



with the digits widely separated at the base, but with the tail almost 

 unchanged ; I. americanvs has a tliick hand with fingers widely sepa- 

 rated at the base, and with the tail enormously dilated towards its 

 distal end. The basal tooth of the pectines is never enlarged ; but in 

 the female of several of the American forms, such as I. americnnus, 

 I. androcoftoides, and I. insignis, there is, at the base of the pectcn, a 

 distinct rounded lobe projecting over the basal tooth. 



Isometriis, so far as geographical distribution is concerned, appears 

 to be the dominant genus of the family, and, as might be expected 

 from its wide range, it varies greatly in structure. Yet in the sum 

 of its characters it appears to come nearest to the ancestral form ; 

 for from it by slight modifications most of the genera of the family 

 can be derived. Thus in Australia it appears to have given rise to 

 Isometroides, in America to Centrurus, in Africa to Butheolus and 

 Rhoptrurus, the latter genus leading on towards Buthus. 



Keasons ^ have already been given for regarding the genus Phassiis 

 as synonymous with Isometrus, on the ground that it was based 

 upon a character belonging to the male of a certain species of this 

 last-named genus. 



With regard to Androcottus it may be said that there is nothing 

 in the diagnosis to warrant the separation of the type as a genus 

 distinct from Isometrus. The fusion of the inferior keels on the 

 second, third, and fourth caudal segments, the character upon which 

 it. was founded, exists, although apparently to a slightly less extent, 

 in /. androcottoides, and varies considerably within the limits of the 

 species. 



Genus Isometroides, Keys. 



Isometroides, Keyserling, Arachn. Austral., Scorpiones, p. 16, 

 pi. ii. figs. 3&4 (188.i). 



Hab. Australia. 



A genus closely allied to Isometrus, differing, in fact, only in the 

 form of the tail, the vesicle being very slender and without a spine 

 beueath the aculeus, and the fifth caudal segment being deeply punc- 

 tured and without keels on its under surface. 



Two species only have been made known, both being figured and 

 described in the above-cited work. Of one of these speces, 7. vesms, 

 the British Museum possesses a single specimen, from Port Lincoln. 



Genus Centrurus. 



Centrurus (Hemp, and Ehrb.), Thorell, Etudes Scorp. p. 9. 

 Rhopalurus, id. ibid. 



Had. America. 



This genus differs from Isometrus only in the armature of the 

 digits of the chete, the space between the large lateral teeth on each 

 side being occupied by a small row of smaller teeth arranged slightly 

 obliquely, although, roughly speaking, parallel to the long axis of 

 the digit. 



1 Ann. Nat. Hist. (6) iii. p. 55 (1889). 



