CRUSTACEA— OSTRACODA. 333 



V. Anomalocaris Whiteaves. 



These bodies resemble the segmented abdomen of a branchiopod, 

 each segment with a pair of lamellate appendages which are gently 

 rounded; posterior terminal segment with three pairs of spines; 

 no carapace known ; of doubtful affinities. 

 9. A. canadensis Whiteaves. Cambric. 



Body from 9-13 segments, exclusive of caudal segment. (Type 

 of genus.) 



Middle Cambric (Stephen formation) of British Columbia, 

 abundant. 



Subclass Ostracoda Latreille. 



Small Crustacea, indistinctly segmented and completely enclosed 

 in a horny or calcareous bivalve shell. The valves are joined 

 dorsally by a membrane; they are either in contact, or overlap. 

 In specialized types a denticulate hinge structure is present. The 

 valves separate along the ventral side and ends. The shell cor- 

 responds to the carapace of other crustaceans. 



The surface of the shell may be smooth, or variously modified 

 by tubercles, lobes and sulci; an eye tubercle, situated in the 

 anterodorsal region is often present. The surface may be pitted 

 or reticulated by a regular net- work of lines ; or it may be ridged or 

 striated by horizontal, more or less branching and uniting ridges. 

 Nodes are frequently found, either of indefinite outline or as 

 regularly and sharply defined tubercles. A characteristic feature 

 of many groups is the development of one or more vertical grooves 

 or sulci. Sharp lines and ridges, parallel to the ventral margin or 

 otherwise disposed are found in many genera; a marginal expan- 

 sion or flange, often overhanging the contact margin of the valves, 

 is not infrequently found. 



The valves may be of equal size, or unequal, the larger overlap- 

 ping the smaller lid-like all around, or ventrally. In most Palaeo- 

 zoic genera the hinge line is formed by the straight dorsal margin 

 of the valves, while in other cases this margin is rounded, the true_ 

 hinge being below it. 



The orientation of the valves is a simple matter when an eye 

 tubercle is present, as this always marks the anterodorsal end. 

 The thicker end of the shell is, as a rule, the posterior, but this is 

 not absolute. When the two ends are unequal, the one showing 



