276 CRUSTACEA OF THE THIRD TANGANYIKA EXPEDITION. [Mar. 5, 



why the same is not true of the Brachyura. An explanation is 

 really not far to seek. All the Macrura concerned are wholly 

 aquatic types, while among the Brachyura we have to institute 

 comparisons with forms which have partially accustomed them- 

 selves to a terrestrial existence. Such a profound change in 

 habits must produce an effect which, in comparison, would dwarf 

 any modification brought about within the limits of a single 

 medium. 



There is gradually being accumulated a mass of information 

 concerning the other animal groups inhabiting Tanganyika, and 

 in nearly every case it is fovind that the forms are to a large 

 extent endemic and, moreover, very distinct and highly modified. 

 The explanation of this fact will be equally the explanation of 

 the remarkable character and variety of the Tanganyika Crab- 

 fauna, when compared with that of the other great African lakes. 

 There seems little doubt that it is to be found in a growing 

 divergence taking place in the lake during a prolonged period of 

 isolation. 



EXPLANATION OF THE PLATES. 



Plate XVI. 



Fig. 1. Fotamon {Potamonautes) orhitospinus (p. 259). Adult male, general view 



from aijove. Nat. size. 

 Fio-s. 2-7. Series of tigm-es of Flati/theTplivsa and Fotamon to show the frontal 



region and illustrate the relations between the front, descending process, 



and antenna (p. 267). 

 Fig. 2. Flati/tJielpJmsa maculata. X 5. 



3. Flatiftlielplmsa conctilcata. X 5. 



4. Fhdiithelpliusa armata. X lo- 



5. Fotamon {Farathelphusa) niloticiis. X l^-. 



6. Fotamon {Potamonautes) platynotus. X 2. 



7. Fotamon {Potamonautes) perlatus. X 2. 



PXATE XVII. 



Fig. 1. Fotamon [Potamonautes) plati/notus (p. 264). Adult female, general view 

 from above. Nat. size. 



2. Platijthelplmsa concidcata (p. 273). Male, general view from above. X 3. 



3. Fotamon {Potamonautes) pJatynotus (p. 264). Larger chela of adult female, 



to show nature of dentation. X li. 



4. Flatythelpliusa conculcata (p. 273). Larger chela of male, to show nature 



of dentation. X 5. 



5. Platythelphusa maculata (p. 271). Chela of female, to show nature of 



dentation. X 5. 



6. Flatythelpliusa maculata (p. 271). Abdominal region of a female, from 



below , to show the large size of the abdomen, and the degree to which 

 it covers the sternum. X 3. 



Feference Letters. 



ant. 1. Antennule. I ex.orh. External orbital spine. 



ant. 2. Antenna (second segment). s.so. Sub-orbital spine. 



d.pr. Descending process of front. I t.so. Sub-ocular tooth. 



