I. '08. 6 



division at or about this line. Although for museum purposes 

 it certainly seems necessary to define some western limit to 

 the British and Irish area, it is evident that its creation is of 

 purely local interest and offers no assistance to the study of 

 distributional problems. 



Fifty-four species ^ of Decapoda Natantia are known from 

 British and Irish waters, and forty-seven of these have been 

 fomid off the Irish coasts. ^ 



Ten species are practically restricted to the littoral and 

 laminarian zones, which extend from high water mark to 15 

 or 20 fathoms. These are : — 



Hippolyte varians. Leander adspersus. 



Hippolyte prideauxiaim. Leander squilla. 



Spirontocaris Cranchi. Crangon vulgaris. 



Athanas nitescens. Philocheras trispinosus . 



Leander serratus. Philocheras fasciatus. 



Hippolyte varians, Leander squilla, and Crangon vulgaris 

 are not infrequently found in brackish ditches where the water 

 is of low salinity, while Palaemonetes variants occurs abun- 

 dantly in water that is only slightly brackish. 



Several species, such as Pandalus Montagui and Pandalina 

 hrevirostris , may be reckoned as visitors to the laminarian 

 zone, recurring there regularly at definite seasonal periods. 



Beyond the laminarian zone there is no distinct line of de- 

 marcation in the fauna, the species changing by almost im- 

 perceptible gradations until the greatest depths of the Atlantic 

 are reached. 



The large majority of the British Decapoda Natantia live on 

 or very close to the bottom. The following species have, how- 

 ever, been taken under circumstances which afford the clearest 

 proof that they are free-swdmming^ : — 



Amalopenaeus elegans. Parapasiphae sulcatijrons . 



Sergestes rohustus. Acanthephyra purpurea. 



Sergesies arcticus. Acanthephyra dehilis. 



Pasiphae sivado. Hymenodora glacialis. 



Caridion Gordoni. 



P. sivado and C. Gordoni occur constantly on the bottom, 

 but are nevertheless sometimes found in midwater, usually in 

 soundings of no considerable magnitude. The seven remain- 

 ing forms are, as a general rule, bathypelagic, but S. rohustus 

 and A. purpurea, having been found in the stomachs of fish 



1 Excluding the following three species, which have as yet only been 

 taken near the Channel Islands : — 



Lysmata seticaiidata (Risso), Norman, 1907. 

 Hippolyte gracilis (Heller), Walker, 1899 (see p. 170). 

 Anchistia scripta (Risso), Norman, 1907. 



2 An Index to the genera and species mentioned in this paper will be 

 found at p. 179. 



^ Three other species also probably occur in midwater — Paaipha'e tarda 

 and the two species of Ephyrina, E. Hoskyni and E. Benedicti. 



