numerous in a batch. They are carried by the mother in the usual way of 

 crabs, and, so far as is known, are hatched not as zoseas, but in a much more 

 advanced stage, and the young stay in the mother's brood-pouch until they 

 have attained the adult form and a considerable size. 



Differentiation of Species and Varieties. — Owing to their frequent 

 variability, the definition of species in this group is peculiarly difficult, and 

 here, as everywhere, unanimity upon the species question seems beyond 

 attainment. 



As to the causes of variation, one may, without offence I hope, make the 

 trite surmise, that in an assemblage of species many of which have a very 

 considerable range of horizon and are moreover tied to a perpetually changing 

 medium— for nothing can be much more inconstant than the conditions of a 

 stream or of a pool of fresh water in the tropics — slight vicissitudes of existence, 

 capable, however, of leaving their mark, may be unusually frequent. 



In discussing the question of variation and the difficulties which it presents 

 to the systematist, the easiest way will be to enumerate the " points " by which 

 species among Indian Potamonidcv are judged, and theu to consider the 

 constancy of each point. 



The points looked to by the systematist may be enumerated as follows, so 

 far as Oriental species are concerned :■ — 



1. Carapace: its shape, its breadth, its depth and dorsal convexity, and its 

 surface sculpture and "areolatiou." 



2. " Cervical groove " : its course or curve ; its depth, distinctness, and 

 completeness. 



3. Front : its breadth, its degree of declivity, and its shape. 



4. Orbit : its shape, the size and sharpness of the external orbital angle, 

 and the presence or absence of a gap between that angle and the lower border 

 of the orbit. 



5. Antero-lateral borders of carapace : their length, their cmwe or convexity, 

 their sculpture, and the size of the lateral epibranchial tooth. 



6. Epigastric crests: their size and distinctness, and their position Avith 

 regard to the post-orbital crests and to the front. 



7. Post-orbital crests : their sharpness and distinctness, their position with 

 regard to the epigastric crests, their relation to the cervical groove, and their 

 relation to the lateral epibranchial spine. 



8. Abdomen of adult male: its general shape, and shape and breadth of 

 the 6th and 7th segments. 



9. Ante mial flag el lum : its degree of development. 



10. Mandibular palp : form of the terminal segment, whether simple or 

 cleft into two lobes. 



11. External maxillipeds : length of their exopodite, presence or absence 



