310 ORTMAXN — DISTRIBUTION OF DECAPODS [Aprils,. 



two subfamilies within the same family, Potamonidce, which has 

 never been disputed.^ Consequently the idea suggests itself that 

 both subfamilies have a common origin, or have descended the one 

 from the other. Transitional forms between them are not known ; 

 this, however, is not astonishing if we consider their geographic 

 isolation. 



The present writer has called attention to the presence in Cen- 

 tral Africa of a group of Potamon, v*^hich he has designated as the 

 subgenus Acanthothelphusa. These species have been united by 

 others with Parathelphiisa, which classification we do not consider 

 to be correct. Although these species are very poorly known, it 

 seems impossible to unite the type-species of Acantliothelphusa 

 (from the Nile) with Parathelphusa^ and it would be well to 

 examine the other species more closely with a view to their possible 

 relation to the American Potamocarcinince. 



Whether this prove to be so or not, this much is unquestionable, 

 that the West African PotamonincB are geographically most closely 

 approached by the South American PotaiJiocarcinitue^ and thus a 

 former connection of the respective parts ^ West Africa and npi'thern 

 South America, is suggested (see Ortmann, 1901, p. 1291). 



4. Subfamily: Trichodactylince. (See Fig. 4.) 



Finally, we are to consider the subfamily Trichodactylince, Ortm., 

 which is divided, according to Ortmann (1897, p. 298), into two 

 genera, Trichodactylus Latr. and Orthostoma Rand., which latter 

 generic name, however, is to be abandoned as preoccupied. Its 

 place is to be taken by Sylviocarcinus or Dilocarcinus M.-E,, 1853. 

 But even Trichodactylus and Dilocarcinus (in its largest sense in- 

 cluding Sylviocarcinus, according to Ortmann, and being identical 

 with Orthostoma^ are not always sharply defined, and, further, the 



^ According to Ortmann {Zool. Jahrb. Syst„Vo\. vii, 1893, p. 430), the Thel- 

 phusidcB {Pot a VI 071 idee) are possibly derived from MenippidcB — i.e., primitive 

 Xanthida (in Alcock's sense). They are primitive Cyclometopa, which, how- 

 ever, in certain characteristics, probably connected with their liabits^ are more 

 highly and abnormally developed, and exhibit (due to convergency ?) similarities 

 to the Catometopa. 



Alcock {Jour. Asiat. Soc. Bengal, V. Iviii, Part 2, No. i, 1899, p. 3) is inclined 

 to regard the Thelphusidce as descendants of the Oziince or Eriphiince (higher 

 Xanthidce'), and takes them for very highly specialized Cyclometopa. 



Both views agree in that the family Xanthiidce is supposed to be the ancestral 

 stock of these freshwater crabs. 



