ContiHhutions to Inveriebrate Palseoniology. 565 



The number of segments here allotted to any given genus indi- 

 cates the maximum number of naked segments known; some of 

 them contain species having a smaller number, and in some a much 

 greater number exists, some of which are concealed within the 

 carapace. Thus Ceratiocaris is known to possess in one species 

 fourteen segments in the abdomen, only six of which are naked. 



The genus Dithyrocaris McCoy, is described as having three 

 longitudinal ridges. This feature is seen only when the two 

 valves are pressed open as in McCoy's example, so as to present the 

 appearance of one large plate; in which case the hinge line forms 

 the middle ridge. 



The third or rostral plate in Peltocaris, Caryocaris, Discinocaris, 

 and Ajytychopsis would appear to be quite analogous to the small 

 rostral plate seen in Ceratiocaris, and supposed to exist in Dithyr- 

 ocaiHs, and perhaps some others, but which is usually absent. It is 

 possible many of the forms may have possessed this rostral plate, at 

 least among those that are deeply notched in front when the valves 

 are spread open. In this case they would as properly be considered 

 as having three plates in the carapace as those grouped under 

 section 7. The forms of this section are usually found with the 

 carapace spread open on the rock, and are then circular and discoid, 

 but when in their natural position would have been more or less 

 roof-shaped. 



Colpocaris Meek presents some features that raise a question 

 as to its true atfinities. The longitudinal crenulated line and the 

 inflection of the supposed ventral border do not seem to be properly 

 understood ; and I am of the opinion they may belong to a different 

 group of Crustaceans. 



Ecliinocaris sulileTis. 



Plate XII, figs. 12-14. 



Echinocaris sublevis Whitfield, Am. Jour. Sci. and Arts, 3d series, vol. 19, p. 36, 



1880. 



Carapace obliquely subovate iu general outline, tlie height equal to two- 

 thirds the length, widest and deepest behind the middle, the posterior portion 

 projecting obliquely backwards and downwards beyond the extremity of the 

 hinge-line ; dorsal-line straight, forming a hinge-line two-thirds the length 

 of the valve ; outer margin of the valves, except on the dorsum, bordered by 

 a narrow, slightly raised and thickened rim ; anterior border nearly vertical 

 from the extremity of the dorsal line, for about one-half the width of the 

 valve, except a very slight rounding backward to the hinge-line above ; below 



