1890.] OF THE FAMILY Bl'THID.C. 117 



denticles, all the arrangements met with throughout the family are 

 easily derivable. The first modification that presents itself results 

 from" the assumption of an obliquely transverse position by the pos- 

 terior tooth or two posterior teeth of each row. Thus arises the 

 " external series" of Dr. Thorell. The internal series results, I 

 believe, from the separation of the anterior tooth of each series from 

 the rest ; this separation is sometimes carried to such an extent 

 that all connection between the tooth and the series from which it 

 arose is lost. 



If this view as to the original disposition of the denticles is correct, 

 the arrangement seen in some species of Isometrus is that which 

 comes nearest to the primitive plan. Thus in, e. g., I.messor^ the 

 anterior tooth of each series is enlarged, but not isolated, and the 

 posterior tooth has altered its position, so that with that which pre- 

 cedes it it forms a transversely set pair : in /. insiffnis the anterior 

 tooth, although still in the same straight line with the rest of the 

 series, is separated by a measurable interval from it, and in Lepreus 

 fscheri the anterior tooth has shifted so much forwards that it is on 

 a level with the anterior end of the row in front of the one from which 

 it originated. 



Genus Lepreus, Thorell. 

 (Plate XIV. figs. 2-4.) 



Lepreus, Thorell, Etudes Scorpiol. p. 8. 



Hab. S. Africa. 



Immovable digit of chelicerae unarmed beneath. The external 

 series of teeth on the chela? is formed by the bending outwards in a 

 direction nearly at right angles to the axis of the digit of the two or 

 three posterior terminal teeth of the median rows ; the internal series 

 by the separation (greater or less, as the case may be) of the anterior 

 terminal tooth. 



The cephalothorax is not distinctly keeled ; the tergites always 

 have one median keel, and in a few cases two lateral short keels ; the 

 caudal keels may be well developed or absent, and there may or may 

 not be a spine beneath the aculeus. 



The tibiaj of the two posterior legs are spurred. The basal pectinal 

 tooth in the female is (? always) enlarged. 



In the arrangements of the denticles on the chelos the species of 

 this genus vary considerably. Thus in L.Jischeri, var. nigrimanus, all 

 the teeth of the internal series have moved so far forwards that each 

 is on a level with the anterior extremities of the row distal to the one 

 from which it originated. Whereas, in specimens of i. occidentalism at 

 the proximal end of the digit each of the separated teeth is about 

 equidistant from the anterior end of its original series and from the 

 corresponding end of the series distal to this last ; but in the middle 

 and distal half of the digit each tooth moves forward and approaches 

 close to the anterior extremity of the series distal to the one to which 



^ I have no object in selecting this species ; it happens to be the first that 

 comes to hand. 



