Report on the Crustacea Scliisopoda of Ireland. 235 



Mysidopsis hibernica, Norman. 

 PI. XVI., figs. 4, 5. 



Survey. — Station 115, off the Skelligs, 62 to 52 fathoms, mud and sand. 

 August 20th, 1890. 

 L^ Previous Record. — Valentia (A. M. N.*). 



The species has hitherto been known from a pair of examples captured by 

 Norman in Dr. Jeffrey's yacht " Ospre)^," at Valentia in 1870. No note was 

 made of the "circumstances as to the dei^th, etc.," under which they were obtained, 

 so that our record furnishes the first exact information on this point. The length 

 is given by Norman as 15 mm. Our solitary example is considerably smaller; 

 and, if only young examples were present at the time we were fishing, many may 

 have escaped through the meshes of the mosquito-net bag. As usual, the anterior 

 appendages are rather defective, the antennal scales having disappeared, and the 

 antennules being more or less denuded of sette. Hence the characters of these 

 appendages are not available for specific diagnosis. The telson and the inner 

 uropod (figs. 4, 5) are, as we think, in sufficiently close agreement with Norman's 

 diagnosis {Ann. Mag. Nat. Hist., s. 6, October, 1892, p. 165, pi. ix., figs. 2, 3, 4) 

 to warrant us in referring the specimen to 31. hibernica. It will be noticed, how- 

 ever, that the apex of the telson diverges slightly from the type. In Norman's 

 figure there is shown but a slight emargination of the posterior border between 

 the inner pair of terminal spines, whereas Mr. Green's drawing shows that the 

 Skelligs specimen has a distinct notch in this position ; while the terminal spines 

 (the longer of which have lost their points) are by no means symmetrical. Slight 

 variations and abnormalities of this structure must be quite familiar to every 

 student of the family. If Norman's figures 2 and 3 {loc. cit.) are drawn to the 

 same scale, the telson must, relatively to the length of the inner uropod, be rather 

 shorter in the Skelligs example than in the type. We believe that we have 

 evidence, from the analogy of other members of the family, that such a difference 

 is often explicable by the size of the specimens, the length of the telson tending 

 to increase with age. In our specimen the lateral margins of the telson have 

 fewer spines than are shown in Norman's figure, as, indeed, might be anticipated 

 from its small size {cf. especially Schistomysis spiriMs). 



The typical structure of the telson cannot be held to be certainly known until 

 more specimens have been examined. Although the asymmetry of the Skelligs 

 example suggests abnormality, it is quite possible that a notch in the jjosterior 

 border is more usual than a simple emargination. 



