SPHiEBOMID, AND ON DTNAMENE BUBEA AND VIKIDIS. 149 



whatever the cause may be, it is very certain that many examples of 

 Idotea and Dynamene correspond most closely in hue to the sea- 

 weeds among which they are found. I have two small specimens of 

 Idotea which are symmetrically banded with dark brown on a light 

 ground. The smaller of the two has the extremity of the tail, or 

 pleon, not quite so round as that of I. parallela, but without any 

 apical tooth or cusp, indications of which are generally present 

 even in very minute specimens of tricuspidata. The other (fig. 

 12) must be assigned to hhtea pelagica, unless that is itself only a 

 variety of tricuspidata. 



Among the Asellidae, Jcera alhifrons has many variations of hue. 

 Messrs. Bate and "Westwood say of it, " the general colour is ashy, 

 but very much varied in its shades in dried specimens, with the 

 front of the head whitish." In fact, however, the difi'erences belong 

 to living examples, which may be had ashen-grey, light brown, 

 dark purplish brown, purple and green-banded, and plain green. 

 Among the Aerospirantia that which comes nearest in general 

 resemblance to Dynamene is Armadillo vulgaris, a species " subject 

 to great variation in the amount of its pale markings, which has 

 led to the establishment of a great number of supposed species." 

 To this remark it may be added that not only do the pale mark- 

 ings vary, but also the ground-colour, which may be dark steel- 

 grey, or bright brownish red, or black, or even, though rarely, 

 creamy white. 



Colour, then, it will be seen, is an insufficient basis for specific 

 distinction among crustaceans, at any rate in the groups to 

 which allusion has here been made. 



Passing on to the other differences which have been noted be- 

 tween D. rvhra and D. viridis, we find the one said to be narrowly 

 ovate and the other broadly ovate. This, however, is a character 

 which seems to depend on the age and size of the individual. In 

 Idotea, tricuspidata the variations in the breadth of the body com- 

 pared with that of the tail are very considerable ; but one would 

 no more think of specifically separating the broad and the narrow 

 examples than one would of making a fat man a distinct species 

 from a thin one- There is, moreover, a peculiarity occasionally 

 to be observed in Dynameiie, whether red or green, and also in 

 D. Montagui, which would seem decisive against the use of 

 breadth as a specific character; for examples may be found of 

 which the head and first four segments of the body are narrow, 

 while the remainder of the body and the pleon, or tail, are broad 



U* 



