AND OTHER BRITISH PRAWNS. 115 



described the colour of this species, and has drawn attention to 

 the orange colour of the chelae as a striking character. 

 Length. Female 29-43 mm. Male 18-25 mm. 

 Mandible. — Palp absent. 



Rostrum. — The rostrum is narrow and straight, or even may 

 be somewhat depressed, and it is armed with a variable number 

 of teeth. I have examined a large number of specimens from 

 Norfolk and from other localities, and find that the formula | is 

 by far the most usual. A small apical tooth is present in more 

 than half the individuals. Four or six dorsal teeth are not 

 uncommon, but it is exceptional to find more or less than two 

 ventral teeth. Only one tooth is situated on the carapace behind 

 the eye. 



The number of rostral teeth has long been known to be variable, 

 and this variation has been studied'by Weldon *, Brozek f and 

 others. The latter has made a statistical study of the numbers 

 of teeth in specimens from various localities in fresh and brackish 

 water, and concludes that the fresh-water form from south 

 Europe has on an average a larger number of dorsal teeth than 

 the brackish northern form. His figures are as follows : 



Plymouth (Weldon) 4-3 



Lago di Castello (Italy) ... 4.9 



Skutari 6-4 



Montenegro 5-3 



Monfalcone (Istria) 5-7 



As in a later paper he has given the frequency for 134 specimens 

 from Copenliagen to have been 6-22, there seems to be no ground 

 for supposing thatthe number of teeth has anything to d*o with 

 geographical situation or salinit}'. 



It is, however, not improbable that a statistical study of the 

 rostral teeth based on a large material would show constant 

 local variations, since many populations of this species must be 

 isolated for long periods and subject to intense selection. The 

 result is hardly likely to justify the great labour necessary, but 

 it may be of interest to summRrize such facts as I have collected 

 bearing on this point (Table II.). The figures given in this 

 table express the frequency of occurrence of "various numbers of 

 dorsal teeth as percentages of the individuals examined. It 

 should, however, be mentioned that Weldon's figures alone are 

 dra.wn from a really adequate material (915 specimens). So far 

 as the figures go, they show that six dorsal teeth is far more 

 frequent on the East Coast and in Scotland than it is at 

 Plymouth, though the examples from the Stour are an appai-ent 

 exception. The range of variation is also less. 



Antennule. — The shorter flagellum is about four-fifths, or 



* Journ. M. B. A. n. s. i. p. 459 (1890). 



t SB. K. Bohm. Ges. Wiss. Jg. 1907, 1909, and 1912. 



8* 



