10 



Gr. 0. SARS. 



[Xo. 1. 



II. 



Morphology and Anatomy, 

 A. The Shell. 



(Pl. 1, figs. 1, 2.) 



The shell, or more properly the carapace, is composed, as 

 in the other forms of this sectiou of Phyllopoda, of two equal- 

 sized valves, connected along the dorsal line and open more or 

 less inferiorly and anteriorly (see Pl. III, fig. 1). The val ves. 

 however, admit of being completely closed. so as to almost 

 enclose the whole animal hermetically within the cavity of the 

 shell. This is effected by the aid of a strong muscle, the adductor 

 of the shell, which passes transversely from one valve to the other. 

 its insertion causing a rounded area at the anterior part of 

 each valve, a little below the umbones, distinctly seen in fresh 

 specimens (see Pl. 1, fig. 3; Pl. II, figs. 1, 2; Pl. VIII, figs. 1,4). 

 Any specialized hinge I have failed to detect; the valves would 

 seem to be simply soldered, as it were, together along the dorsal 

 line as in the Cladocera, though meeting at an acute angle. The 

 form of the shell, as seen from the side, is somewhat different in 

 young and in adult specimens. In the latter, it is (Pl. 1, fig. 1) 

 irregularly rounded, witta the height well-nigh as great as the length 

 and with distinctly prominent, rounded umbones, occurring close 

 to the anterior extremity. Each of the valves becomes thus very 

 inequilateral, the anterior part, in front of the umbones, being 

 rather short and also much narrower than the greatly expanded 

 posterior part. The part of the dorsal line lying behind the 



