54 Mr. R, I. Pocock on Isoraetrus americanus {Linn.). 



was described and figured by De Geer (Mem. vii. p. 344, 

 pi. xb'. figs. 5-8). De Geer's specimen was examined by 

 Dr. Tliorell and pronounced to be specifically identical with 

 a scorpion known as Atreus ohscurus of Gervais — a scorpion 

 recorded from Columbia, described in Arch. Mus. iv. (1844) 

 p. 219, and figured in Exped. de Castelnau, kjcorpions, pi i. 

 fig. 3. At the end of Gei'vais's description, on p. 220, 

 reference is made to a specimen of ohscurus belonging to 

 M. Goudot. 



In 1846 M. Goudot's collection came into the possession 

 of the British Museum. One of the specimens in this collec- 

 tion agrees precisely with the description and figure of oh- 

 scurus, and is, moreover, ticketed, apparently by either Ger- 

 vais or Goudot, with that name. 



An examination of this speciihen corroborates Dr. Thorell's 

 determination of the synonymy of ohscurus with americanus. 

 But my conclusions with regard to the sexes of the two 

 typical specimens are by no means in accordance with those 

 of that author. On p. 90 of his well-known work he re- 

 marks, "Sc. europomsj De Geer, mas est Sc. ohscuri, Gerv." 

 The grounds for this belief I do not know ; but there are 

 several reasons which lead me to think that the two speci- 

 mens are of the same sex, and females. In the first place, 

 judging from the figures, there is bet\veen the two no differ- 

 ence which by analogy can be regarded as sexual. In the 

 second place, both specimens present that lobate dilatation of 

 the base of the pectines, which is, I believe, a sexual cha- 

 racter a})pertaining to the female alone. This belief is based 

 (1) upon the discovery of ova in specimens of an allied 

 species presenting this pectinal peculiarity, and (2) upon 

 the existence in the collection of the British Museum 

 of a number of specimens of a species of Isometrus (taken 

 at the same time and in the same place) in some of which 

 the pectines are lobate while in others they are not ; 

 and, further, those in which the pectines are not lobate 

 differ from those in which they are lobate in other cha- 

 racters which by analogy belong to the male sex. These 

 characters are — a wider tail, a wider hand, and a wider space 

 between the fingers when closed. The specimens, then, pre- 

 senting these last features there are good reasons for looking 

 upon as males ; and since the females of these differ only in 

 minor particulars from the co-type of ohscurus, I think there 

 cannot be the smallest doubt that the two forms represent 

 the sexes of Isometrus americanus (Linn.). 



But another species described by Gervais, of which a co- 

 type is also in the British Museum, differs from ohscurus in 



