456 



GEOLOGY OF OHIO. 



It will be readily seen from the above synopsis that Echinocarh 

 differs materially in the features of the carapace from all the other 

 genera enumerated. The features of the abdomen and caudal parts are 

 not as reliable, but are somewhat distinctive as may be seen by the fol- 

 lowing table of comparison. (A mark of interrogation indicates that' 

 the parts are unknown or only partially known.) 



Genus 



The number of segments here allotted to any given genus indicates 

 the maximum number of naked segments known; some of them con- 

 tain species having a smaller number, and in some a much greater num- 

 ber 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 longi- 

 tudinal 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 

 Aptychopsis would appear to be quite analogous to the small rostral plate 

 seen in Ceratiocaris, and supposed to exist in Dithyrocaris, 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 affinities. 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. 



