KONGL. SV. VET. AKADEM1ENS HANDLINGAR. BAND. 22. N:0 7. 285 



»Affinis hjec species 1) Onisco Cicadas Oth. Fabricii, a quo tamen difTert capitis pedumque 

 forma, colore et magnitudine; 



2) Onisco Medusarum O. Fabricii, cuius tameu oculi lineares, arcuati, coerulei, lateribus 

 frontis innati, nimis discrepant. Cum hac utraque Gammarus Libellula peculiare genus constituat, 

 in familias huius descriptione monographica arctius definiendum. 



Unicum huius animalculi specimen die vicesimo nono mensis Iunii anni prasterlapsi accepi 

 vivum e mari prope Insulam Ian Meyen protractum, plura autem mense insequente mortua in 

 stomacho Procellariae glacialis reperi, integra quidem et digestione vix laesa, nisi quod pedum 

 subtilissima pubes detrita esset. 



Inter haec iuvenilia quoque, dimidire reliquorum magnitudinis, cfeterum simillima illis.» 



The new species of Mandt and Lichtenstein was however forgotten for many 

 years by carcinologists until A. Goes in 1865 restituted it as Themisto libellula, Mandt. 

 During the interval the species had received new names, as in 1838, Themisto arctica, 

 Kroeyer, and Th. crassicornis, Kroeyer, and as early as in 1835 it had been identified with 

 Guerins Themisto Gaudichaudii, thereby being for the first time placed in the genus to 

 which it really belonged. In 1887 I proposed the name Euthemisto Nordenskioldi for 

 animals which I after further researches have found to be only younger and less developed 

 specimens of Euthemisto libellula; the characteristics on which the supposed new spe- 

 cies was founded have proved to be of no specific value, as they change with the growth 

 of the animal; thus for instance the head is much larger in the young than in the adult 

 animal, the carpal process of the second pair of perasopoda is shorter, and not only the 

 length, but also the shape of the fifth pair, changes with the age. 



Owing to its size the adult animal is one of the giants of the group, being inferior 

 only to some species of Thaumatops, and, if length be considered, also to Xiphocephalus 

 armatus. Some of the species of Lanceola approach the extreme length of Euthemisto 

 libellula. The females seem to attain a greater size than the males. The largest male 

 I have examined measured 35 mm. in length from the front margin of the head to the 

 apex of the last pair of uropoda. The development of the fifth pair of peraeopoda is 

 liable to great individual variation, and this not always in strict relation to the size of 

 the animal, so that we may find large individuals, females as well as males, with this 

 pair comparatively short, and only a little longer than the next; but on the other hand 

 the characteristic features of the fifth pair are at once recognizable, namely the breadth 

 of the femur, the elongated tibial process, the strongly developed carpus, and the bundle 

 of spine-like teeth on the front margin of the dactylus; these features are so constant 

 that we find them even in young ones, a few days or even one day old. This is the 

 reason also why I have maintained the generic distinction between Euthemisto and 

 Parathemisto. The carpal process of the second pair is somewhat shorter in young 

 specimens than in the adult, but even in the very young it is always more than half as 

 long as the hind margin of the metacarpus. 



