On Siriclla arninta and S. frontalis. 151 



XX. — On Siriella armata {M.-Edw.) and the reputed Occur- 

 rence of S. frontalis [M.-Kdw.) in British Seas. By 

 E. W. L. Holt and W. I. Beaumont. 



Siriella armata (M.-Edw.). 



Siriella {Cynthilin) frontalis, Norman, Ann. & Mag'. Nat. Hist, 1802, 



X. pp. 152, 153 ; W. Garstaug, Journ. ]\Jar. J'iol. Assoc, iii. p. 221. 

 Mysis producta, Gosse, Ann. & Mag. Nat. Hist. ser. 2, vol. xii. p. 156. 



In examining a considerable collection of Siriella taken at 

 various parts of the coast from Start Bay to Falmouth, we 

 have been unable to find a single male that can be referred to 

 S. frontalis^ although that species has been recorde 1 from 

 Plymouth by Norman and by Garstang, in each case from a 

 single specimen. S. frontalis {Pseudosiriella frontalis, Glaus, 

 Arbeit. Zool. Inst. Wien, v. iii. p. 6) differs from all its con- 

 geners in having the male organ of the pleopods simple 

 and leaf-like, instead of bilobate and convoluted. In other 

 characters S. frontalis and S. armata are extremely alike, so 

 that there is difficulty in distinguishing females when no 

 males are available, as was at first our case, from existing 

 descriptions. In nearly all our gatherings, however, both sexes 

 are present. All males have the appendages of the pleopods 

 bilobate and convoluted, but in males and fenuiles alike the 

 characters of the telson show considerable variation and 

 appear to us to be of doubtful value in diagnosis. The 

 number of terminal spinules between the large postero-lateral 

 pair of spines of the telson ranges from three to five, while 

 the spinules in the intervals between the spines on the lateral 

 borders of the same organ a])proach in many cases the 

 formula of S. frontalis (Norman, loc. cit.) rather than that of 

 S. armata (Norman, 4th Ann. Report Fishery Board for 

 Scotl. p. 165). Sars has given figures* of the antennal 

 scales in the two species, showing a fairly obvious difference 

 in shape ; in our specimens that appendage, while agreeing 

 best Avith S. armata, shows an undoubted approach to the 

 condition of S. frontalis in some cases. The pereiopods are 

 described as more slender in S. armata than in S. frontalis. 

 Such a character is difficult to seize without comparison of 

 undoubted examples of the two species ; but all our specimens 

 appear referable in tiiis particular to the first-named. 



We were thus irresistibly compelled to the belief that of 



• G. 0. Sars, ' Middelhavets Mysider,' pi. xxxiv. fig. 2 k pi. xxxv. 

 fij?. 4. 



