350 CEYLON PEARL OYSTER REPORT. 



and termed by him facultative dimorphism. SMITH has shown that in males of this 

 species there are at least two breeding periods (" low " and " high " respectively) 

 characterised by well-developed secondary sexual characters, and that between these 

 is intercalated a non-breeding phase (" middle ") in which the secondary sexual 

 characters are not evident. What SMITH has concluded for Inctchus scwpio from 

 statistical evidence, WALTER FAXON had found in a Cambarus reared by him in an 

 aquarium. HAGEN had previously described two types of male Cambarus and con- 

 sidered them to be characteristic of different individuals, but FAXON, observing 

 aquarium-kept animals, found that the two conditions detailed by HAGEN were 

 alternate phases in the life-history of the same individual correlated with the 

 breeding and non-breeding period respectively ; the breeding male with pronounced 

 secondary sexual characters changed by a moult to the non-breeding form with much 

 resemblance to the young. A very beautiful example of facultative dimorphism is 

 added to the above by a series of Mencethius monoceros in the present collection. 

 There is evidence that the same kind of thing is of wide occurrence amongst the 

 Oxyrhynchs. 



The importance of the matter for systematic zoology may be emphasised by reference 

 to Simocarcinus simplex arid S. pyramidatus, one of the very few differentia between 

 which is the cheliped character a difference for which the theory of facultative 

 dimorphism offers an alternative explanation. 



In working over a large collection of crabs, attention is constantly attracted by the 

 considerable amount of growth and of correlation-change which commonly occurs after 

 sexual maturity. 



Certain contractions have been found convenient in the following pages : 



C. = carapace, Ch. = cheliped, W.L. = walking leg, F. = finger (dactylus of cheliped), 

 H. = hand, 1. = length, b. = breadth, Bord. = border, R. = rostrum. Unless other- 

 wise stated, Ch.l. is measured along the morphological ventral border, and is the sum 

 of (1) a straight line uniting the base of the appendage to the distal end of the 

 merus, and (2) a straight line uniting the last-named point to the tip of the fixed 

 finger. In Oxystomata it is measured along middle of posterior surface. 



Measurements are in all cases given in millimetres. 



Colonel ALCOCK'S "Materials for a Carcinological Fauna of India" is indispensable 

 to the student of Indian crabs. I have followed him where possible in matters of 

 nomenclature and classification. 



Space forbids synonymies ; I have in most cases made reference to one good account 

 only of the species in question. A useful list of the literature will be found in 

 KLUNZINGER (1906). The following contractions have been employed : 



A.1.-A.6. = ALCOCK, "Materials, &c.," No, 1-No. 6, in ' Journ. Asiat. Soc. Bengal, 

 1895 to 1900. 



