Variation in Portunus depurator. 221 



in air. From the modification of their appearance, and the measure- 

 ments originally made from them, their identity was not quite 

 apparent, although probable. 



It thus appears that, with the spark, the cyanogen spectrum is 

 nothing like so strongly marked, as is the case with the flame of the 

 gas, only one group of bands being represented, and that when the 

 spectrum is taken in air the cyanogen does not appear, because in 

 all probability the substance is oxidised. 



I have already stated that the formation of cyanogen which yields 

 the characteristic spectrum is a synthetical operation, that compound 

 substances, when burnt in flames, do not, as a rule, emit the spec- 

 trum of the compound, but the spectrum of one or more of the 

 elements of which it is composed, or that of one or other of its 

 products of combustion. 



How then are we to account for the cyanogen spectrum in the 

 flame of burning cyanogen ? 



The conditions under which combustion takes place are these : 

 there is an excess of the gas, the temperature of the flame is exceed- 

 ingly high, and the gas within it is not in contact with a solid sub- 

 stance, hence immediate decomposition does not occur, and the 

 gaseous compound is heated to incandescence. 



" Variation in Portunus depurator" By ERNEST WARREN, B.Sc., 

 Demonstrator of Zoology at University College, London. 

 Communicated by W. F. R. WELDON, F.R.S. Received 

 July 1, 1896. 



The following measurements were undertaken at the proposal of 

 Professor W. F. R. Weldon, and to him I am greatly indebted for 

 many suggestions, and for the kindly help he has always so readily 

 given me. 



The crabs were obtained from the Biological Station at Plymouth, 

 and sent at intervals during a period of about two years, dating from 

 the autumn of 1893. Only males were measured. Seven measure- 

 ments were made on each individual, corresponding to those made by 

 Professor Weldon on the female of Garcinus moenas (' Roy. Soc. Proc./ 

 vol. 54). 



1. Carapace length, AB (fig. 1). 



2. Total carapace breadth, CO'. 



3. Frontal breadth, DD'. 



4. Right antero-lateral, AC. 



5. Left antero-lateral, AC'. 



6. Right dentary margin, CD. 



7. Left dentary margin, C'D'. 



VOL. LX. R 



