100 NEW YORK STATE MUSEUM 



shown in the great carapace of M e s o t h y r a and seen to be 

 present, though less strongly defined, in Rhinocaris. In 

 front of this contact point of the carapace valves is a rostral cleft, 

 the rostral plate filling it and projecting forward beyond the 

 anterior mar^n of the shield. Its form in front has been rep- 

 resented bythe writer as flat laterallyand leaflike, but this may be 

 a shape resulting partly from compression. Some of the new ma- 

 terial seems to indicate that this plate was broader over its ante- 

 rior portion. In Mesothyra, only the separated valves and 

 segments of which have been found in the Ithaca beds at Ithaca, 

 all these features are strongly pronounced. Indeed this genus 

 was founded and separated from Dithyrocaris on the 

 basis of this structure {Paleontology of New York, v. 7) before the 

 same carapace structure had been determined in Rhinocaris 

 save for the presence of the rostrum, which was then looked on 

 as a rostral projection consolidated with the carapace. We have 

 found no reason since the publication of the restoration figure of 

 Mesothyra {Pwleontology of New York, v. 7), for modifying our 

 conception of the structure of this shield. There are other 

 features which justify this conception of Mesothyra as 

 generically distinct from Rhinocaris, and we should find the 

 distinction most emphatic in the extreme development in the 

 former of every structural detail. 



Mesothyra oceani bears a single but immense lateral 

 carina on each valve. No such structure is observable on the 

 original specimens of R h i n. c o 1 u m b i n a, though a carina is 

 present in other species of that genus, sometimes extremely ob- 

 scure. Thus there are certain carapaces from the soft Hamilton 

 shales of the Livonia salt shaft which are like those of R h i n . 

 c 1 u m b i n a but relatively broader and with radial branching 

 grooves departing backward from the eye nodes, on which a faint 

 carina is observable. This form we propose to designate as Rhin. 

 columbina var. livonensis. The carina is likewise pres- 

 ent though faint in Rhin. veneris. In Rhin. scaph- 

 o p t e r a and Rhin. h a m i 1 1 o n i a e it is more decided in its 

 development. 



