| OCR Text |
Show 120 MR. R. I. POCOCK ON THE SCORPIONS [Mar. 18, with the digits widely separated at the base, but with the tail unchanged ; 1. americanus has a thick hand with fingers widely separated 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. americanus, I. androcottoides, and I. insignis, there is, at the base of the pecten, distinct rounded lobe projecting over the basal tooth. Lsometrus, 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 Lsometroides, in America to Centrums, in Africa to Butheolus and Rhoptrurus, the latter genus leading on towards Buthus. Reasons ' have already been given for regarding the genus Phassus as synonymous with lsometrus, 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 lsometrus. 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 I. androcottoides, and varies considerably within the limits of the species. Genus ISOMETROIDES, Keys. lsometroides, Keyserling, Arachn. Austral., Scorpiones, p. 16, pi. ii. figs. 3 & 4 (1885). Hab. Australia. A genus closely allied to lsometrus, differing, in fact, only in the form of tbe tail, the vesicle being very slender and without a spine beneath the aculeus, and the fifth caudal segment being deeply punctured 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, I. vescus, the British Museum possesses a single specimen, from Port Lincoln. Genus CENTRURUS. Centrums (Hemp, and Ehrb.), Thorell, Etudes Scorp. p. 9. Rhopalurus, id. ibid. Hab. America. This genus differs from lsometrus only in the armature of the digits of the chela?, 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). |
