332 On the_ Variations of certain Crustacea. 



no such, arrangement is possible without leaving numerous 

 intermediate forms — evidently, as it appears to me, links in a 

 chain of descent — which cannot be referred with certainty to 

 any genus. Up to a very recent period, the recognized genera 

 of these two families, founded upon characters of hingernent, 

 were as follows : — (1) Jonesia*, the hinge of which is perfectly 

 simple, the edge of the valves being quite straight, and held 

 in opposition by ligamentous tissue ; (2) Gytherideis, in which 

 the edge of one valve projects sharply at each extremity, and 

 articulates with corresponding excavations or notches of the 

 opposite valve ; (3) Gytheridea, bearing on the right valve two 

 knurled or crenulated elevations, which are received into 

 shallow fossee of the opposite valve ; (4) Gythere, which, instead 

 of knurled elevations, bears two strong projecting teeth, arti- 

 culating with corresponding fossse of the left valve, and between 

 the two teeth a bar which is received into a furrow of the 

 opposed valve ; (5) Gythereis, similar to the foregoing, except 

 that the bar and furrow are obsolete ; (6) Bairdia,\ in which 

 the left valve is much the larger, overlapping considerably the 

 edge of the right valve, which is received within it. 



But between the typical forms of Jonesia and Gytherideis 

 there are many gradations of hinge character, no two species 

 being exactly alike in this respect. Many species, instead of 

 having margins either absolutely straight or angularly notched, 

 have more or less pronounced curvatures. In this category 

 may be placed Gythere variabilis, Baird, 0. contorta, Norman, 

 and other species. The genera Gythere and Cytheridea are 

 not, so far as we at present know, so closely interwoven ; still 

 there are several species which are intermediate in character. 

 Gne of the commoner littoral species, Gythere lutea, Muller 

 (G. reniformis, Baird), has hinge-processes which are distinctly 

 crenulated, and many species possess a crenulated bar between 

 the two terminal teeth. Gythere and Gythereis are so inex- 

 tricably united that it is impossible to draw any line of sepa- 

 ration which at all takes into account the character of the 

 hinge, and even if we look to more general peculiarities, the 

 task of discrimination is scarcely more practicable. The genus 

 Bairdia is very distinct indeed in its most strongly marked 

 forms, the great inequality in the size of the two valves, and 

 the peculiar overlapping of the left hinge-margin being un- 

 mistakeable characters. But besides that the tendency to a 



* The genus Jonesia was proposed by myself (Trans. Zool. Soc, vol. v.), 

 but appears to be equivalent to Bythocythere, G. O. Sars, a genus founded 

 on exclusively animal characters. When my paper was written, I was not 

 acquainted with the valuable memoh- of M. Sars, which indeed was probably not 

 printed at the time. 



t The genus Bairdia is related to Cypris, belonging to the family Cyprina, 

 and not Cytherince, with which it ha3 been hitherto classed. 



