4 G. O. Sars. 



Crustacea become sexually mature long before they have 

 attained their full size, and their continued growth apparently 

 undergo, to a certain extent, alterations even in characters 

 which have hitherto been regarded as of fixed specific value. 

 This seems especially to be the case with the bivalvous 

 forms, for which, besides the size of the shell, its form and 

 the number of lines of growth have generally been adopted 

 as specific characters. Now it appears, however, that all 

 these characters are subjected to no small alteration with 

 the age of the specimens. On comparing the figures of 

 Leiotesiheria siliqva given in my 2nd paper with the 3 forms 

 figured on PI. Ill of the present paper, the difference in 

 the external characters of the shell appear, indeed, so pro- 

 nounced, that, according to general practise, one would 

 be very much disposed to regard them as so many different 

 species. Yet I am now of the opinion that in reality they 

 belong to one and the same species, and that the apparent 

 differences are only due to the different development of the 

 specimens. With regard also to the species of A])us, some 

 difficulties present themselves. The body being highly con- 

 tractile, it is not always easy in preserved specimens to state 

 with exactness the relative length of the exposed part as 

 compared with that of the carapace, and the outline of the 

 latter may also vary to a certain extent. In the structure 

 of the several appendages, great uniformity seems to prevail 

 in the different species, and it is scarcely possible to derive 

 any valuable specific mark from them. On the other hand, 

 the number of caudal (not pedigerous) segments seems to 

 afford a very good and easily recognizable distinguishing 

 character; but, as this number is not the same in the two 

 sexes, both male and female specimens ought to be examined 

 in this respect, in order to utilise this character for specific 



