28 ME. E. J. MIEES ON THE IDOTEIDiE. 



Body smootli, moderately convex, and not tuberculated or 

 rugose. Head with the antero-lateral angles very little promi- 

 nent and rounded, the anterior margin very slightly emarginate. 

 First thoracic segment with the antero-lateral lobes subacute 

 and not quite reaching to the eyes. Postabdomen about equalling 

 in length the five preceding thoracic segments ; terminal segment 

 with the sides straight and slightly convergent to the distal 

 extremity, which is usually more or less tridentate, with the 

 postero-lateral lobes (or lateral teeth) rounded and much less 

 prominent than the median tooth, which is obtuse ; there are 

 usually more or less marked indications of a median keel on the 

 dorsal surface of the segment. In other, even adult, examples 

 there are no indications of any except the median tooth. Eyes 

 small. Antennules with the basal joint little dilated, not reach- 

 ing beyond the penultimate joint of the peduncle of the antennas 

 in the adult. Antennae, when retracted, about reaching to the 

 posterior margin of the fourth thoracic segment, with the last 

 peduncular joint a little longer than the preceding; flagellum 

 with not more than 20 joints, and usually about 16 in the 

 adult. Legs slender; epimera of second to fourth segments 

 more or less oblong and reaching to the posterior margin of the 

 segment, those of sixth and seventh segments with the postero- 

 lateral angles acute. Posterior plates of the operculum sub- 

 oblong, longer than broad in the adult. Colour very variable. 

 Length of an adult male does not usually exceed 1-g- inch 

 (SOmillim.), breadth rather more than ^ inch (9millim.). 



The variability of /. marina in regard to its colour, markings, 

 and the length and number of articulations in the antennae, which 

 are generally shorter in the young individual, has been commented 

 on by Messrs. S. Bate and "Westwood, Parfitt, and other authors. 

 The younger examples, in their more convex and narrower body, 

 shorter antennae, and obsolete postero-lateral teeth of the terminal 

 segment, are generally of the form designated by Leach I. pelagica. 

 The largest example in the Museum collection (a male from the 

 Shetlands) is nearly 1-j^ inch in length ; in it the antennules do not 

 reach to the end of the antepenultimate joint of the antennae, and 

 the median posterior tooth of the terminal segment is prominent 

 and elongated. In most of the Mediterranean examples I have 

 seen the median posterior tooth of the term.inal postabdominal 

 segment is very short, in some almost obsolete. These may be 

 designated var. Basteri, Audouin. It may be convenient to retain 



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