AND OTHER BRITISH PRAWNS. 99 



Key for determination of the Sjiecies. 



1. Carpus of 2nd leg much shorter than merus i. serratus. 



Carpus longer than merus 2. 



2. Carpus of 2nd leg longer than chela Falmmonetes varians. 



Carpus shorter than chela 3. 



3. Dactylus of 2nd leg one-third the length of the chela i. squilla. 



Dactylus nearly or quite one-half length of chela 4. 



4. Short flagellum of antennule as long as or longer thai> 



peduncle L. adspersus. 



This flagellum much shorter than peduncle L. longirostris . 



1, Leander SERRATUS (Pennant). 



Colour. — Thorax and abdomen strikingly banded with brownish 

 red, the lines on the thorax running almost horizontally, or 

 obliquely forwards and upwards. Rostrum covered with small 

 red chromatophores. Legs banded with purple and yellow. 



Rostrum slender and greatly exceeding the length of the 

 antennal scales. It is conspicuously upturned and devoid of 

 spines in its distal third. The apex is bifid, and the usual number 

 of spines is 7 dorsally and 5 on the ventral margin. The first 

 spine is situated well behind the eye, and the second either above 

 or slightly behind it. 



De Man {ibid. p. 169) has described three specimens in which 

 the rostrum was of abnormal form, and I have had the opportunity 

 of examining two specimens in the museum of the Marine 

 Laboratory at Plymouth which are of some interest. In both 

 these cases (measuring 57 and 54 mm.) the rostrum is straight, 

 without the upturned toothless portion characteristic of L. serratus, 



the apex undivided. The spine-formula is g and g' 



In both cases the general resemblance of the rostrum to that 

 of L. squilla was rather striking, and they also agreed more with 

 L. squilla in having the short flagellum of the antennule approx- 

 imately equal in length to the peduncle. On the other hand, the 

 form of the second leg, and in one case the palp of the mandible 

 (that of the other was not seen), left no doubt that these were two 

 abnormal examples of L. serratus. 



In quite young specimens up to about 25 mm. the rostrum 

 alone is by no means a safe guide to identity. In the autumn on 

 the South Coast young L. sqioilla and L. serratus are found 

 commonly in rock-pools mingled together, and their separation is 

 not altogether easy. In such specimens the length of the dactylus 

 of the second leg is also an unreliable chai-acter, since this joint 

 in specimens of L. squilla up to 15 mm, may nearly equal the 

 length of the palm, and the adult form is only gradually assumed. 

 On the other hand, the antennule provides, as I believe, a safe 

 means of separating the two species. In such small specimens of 

 L. squilla the free part of the shorter i-amus is much shorter than 



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