1905,] FROM CHRISTMAS ISLAND. 541 



of the former is, in the middle, only half as broad as the ischium, 

 whereas it narrows more anteriorly. The epistome has a different 

 form : in Ptych. pusillus it is barely broader laterally tha,n in the 

 middle, but in Pseudogr. barbatios the posterior margin is strongly 

 arcuate, so that the epistome appears much broader laterally than 

 in the middle. The legs are much alike in both species, but in 

 Pseudogr. barbatus the last three joints of the ambulatory legs 

 are much onore tomentose. Pseudogr. barbatus finally attains a 

 larger size. 



Measurements of the two specimens of Ptych. pusilhos Heller 

 from Christmas Island, in millimetres. 



6. $. 



Distance between the antero-lateral angles of 



the carapace 1 3'5 1 0*5 



Distance between the second teeth 1 7'5 1 3"25 



Distance between the third teeth, i. e. the 



greatest breadth of the carapace 18 13'75 



Breadth of the carapace at the posterior end of 



the latera 1 margins 17 13 



Length of the carapace, in the middle line ... 14'5 11*5 



Breadth of the frontal border 7*2 5*5 



Breadth of the posterior border of the carapace 7*5 6 



Breadth, in the middle, of the ischium-joint 



of the external maxillipedes 2 1*5 



Breadth, in the middle, of the exognath 2*2 1"3 



Horizontal length of the chelai 1 3"5 6'5 



,, ,, „ palm 6 3 



Height of the palm, at the articulation of the 



fingers 775 3-3 



Length of the antepenultimate joint of the 



abdomen, measured in the middle 2'1 



Length of the penultimate joint 2'1 



Breadth of the posterior border of this joint... 4*12 



„ „ anterior _ _ „ „ „ ... 27 



Length of the terminal joint 2*6 



Revision of the Genus Ptychognathus Stimps. 



The genus Ptychognathus., created by Stimpson in 1858 (Proc. 

 Acad. Nat. Sci. Philadelphia, p. 104), and identical with the genus 

 Gnathograpsus A. M.-Edw. 1868, is, at the present time, September 

 1905, represented by the following twelve species : — 



1. Ptychognathus glaber litinips. 1858. c? • 



2. „ ;^«6S'?7Zms Heller 1865. c? 5 . 



3. „ riedelii A. M.-Edw. 1868. c^ ? . 



3 a. ,, ,, var. pilosa de M. 1892. S. 



4. „ pilipes A. M.-Edw. 1868. c? • 



5. „ barbatics A. M.-Edw. 1872. S ? • 



