30 MEMOIR II. 



My sis vulgaris.^' Plate I. This species which with us 

 is probably the most common of any of the Opossum 

 Shrimps, does not appear to have attracted the notice of 

 any Naturalist, a circumstance, either owing to their 

 having been taken for young fry of Shrimps, or to the 

 little attention hitherto given to this tribe of animals. 

 When full grown, they are about one inch in length, trans- 

 lucent, and of a greyish colour. The Clypeus ends in an 

 acuminate point in front -, the anterior scales are of a 

 taper form, and ciliated all round ; the outer laminae of 

 the tail taper to a point, the middle lamina ends in an 

 obtuse point, surmounted by two short spines. They 

 swim with the body in a horizontal position, and abound 

 in the Lee even up to Cork, from the early part of Spring 

 to the approach of Winter j during the still period of the 

 tide at low water, they repose upon the mud and stones 

 at the bottom of the river, and as the tide rises, may be 

 observed forming a wide belt just within its margin, the 

 youngest swimming nearest to the shore, the oldest farther 

 out and in deeper water; they appear to be mostly females, 

 the males being few in proportion. This species contri- 

 butes towards the food of various young fish, from which 

 they frequently escape by springing up out of the v/ater. 



My sis scoticus, the integer of Dr. Leach, who disco- 

 vered this species on the coast of the Isle of Arran, in the 

 estuary of the Clyde, but has not furnished sufficient 

 characters tp distinguish it from the former ; like it, the 

 middle lamina of the tail is without any notch at its extre- 

 mity, but it is a much smaller animal, being but one-third 

 of an inch in length, and different in colour and habitudes ; 

 colour, pellucid cinereous, spotted with black and reddish 

 hrotvn. Females more abundant. He observes, that at 

 low tides near Loch Ranza in the Isle of Arran, the pools 



* Mysis Cauda lamella iutermedia Integra, subulata, margiue spinulosa : 

 lamellis anterioribus acuminatis utrinque ciliatis. 



