150 ON BTNAMENE RUBRA AND VIRIDTS. 



(figs. 9 and 11). The effect to the eye of the perfectly abrupt 

 transition is very curious. The animals exhibiting this forma- 

 tion appeared, when taken, if one may judge from their activity, 

 to be perfectly healthy. It seems possible that these animals 

 may now and then not shed the whole of their integumentary 

 tissues at the same time, and that in consequence the hinder 

 portion is able to expand while the front remains contracted. 

 It is certainly the case that a Ligia oceanica in confineineut 

 thus parted with only a portion of its integuments, those, 

 namely, of the pleoa and the three hinder body-segments. 



The only other point which seems to be depended on as dif- 

 ferentiating D. Q'ubra from D. viridis relates to the little longitu- 

 dinal slit in the terminal tail-segment, which is said in rubra to 

 be "of equal width throughout," or "nearly of equal width 

 throughout its entire length," and in viiHdis to be " widest at its 

 base," or " considerably widened at its base ; the extremities of 

 the sides of the slit sometimes closely approximating or touching 

 each other," with the additional remark that " in the young states 

 the sides of the terminal slit of the tail gape to a considerable 

 width at their extremities." The small slit in question much 

 resembles the slit in the leaf of the sweet-scented Coltsfoot, 

 and probably varies in its proportions in different individuals, 

 much as that proper to the plant in different leaves. 



Neither separately nor together do the differences in colour, 

 breadth of body, and width of the terminal slit seem to be of 

 specific value. Idotea tricuspidata, so often referred to, would 

 give a set of exactly parallel differences, in colour, breadth of 

 body, and length of the apical termination ; and the rule which is 

 perforce admitted in regard to these, cannot fairly be withheld 

 from applying to the case of Dynamene. I propose, therefore, to 

 finite the two so-called species D. rubra and D. viridis under the 

 name of Dynamene varians. There is, however, a Crimean Dyna- 

 mene (= Campecopea versicolor, liathke ; Cymodocea versicolor, 

 Milne-Edwards) which, for all that we can tell from the description 

 in the great French work, may be the same as our English D. va- 

 rians. The description is as follows : — " Corps ovulaire, bombe et 

 lisse. Tete arrondie ; fente du dernier article de I'abdomen un peu 

 elargie a sa base. Habite les cotes de la Crimee." All the cha- 

 racters here given for the E/Ussian apply equally to the English 

 species ; and, as habitat is no guide whatever, our only reason for 

 thinking variant and versicolor distinct is, as far as the distin- 



