102 NEW YORK STATE MUSEUM 



tery in the village of Deruyter/ is distinct from all thes-e forms 

 in the undulating ridge or very faint carina departing from the 

 anterior extremity backward, lying far dorsad of the usual posi- 

 tion of the carina. The ornament on the two sides of this ridge is 

 quite different. As the original figure of this species was but a 

 rough pen sketch, a more finished drawing is here presented. The 

 character of the hinge in this species is still undetermined, 

 though the probability of its being in harmony with that of 

 Rhinocaris is very strong. 



Of the genus Mesothyra we know the carapace structure 

 in the Ithaca species, M. o c e a n i. M. n e p t u n i, from the 

 Hamilton shales at Plainfield (N. Y.) is known only from its tail 

 plate and spines. It seems probable that when more is known 

 of the Hamilton fossil it will prove to be a species closely allied 

 to that of the Ithaca beds. 



Whatever the future may determine as to the propriety of 

 keeping the genera Rhinocaris and Mesothyra apart, 

 these seem now to represent inceptive or progressive structure 

 toward the genus D i t h y r o c a r i s. Though Dithyrocaris 

 is a genus of the longest standing among these, phyllocarids 

 and has been the subject of a recent monograph by Jones and 

 Woodward, we are still somewhat in doubt as to its carapace 

 structure. Up to 1889 Dithyrocaris had been represented 

 and described as bivalvular and having only a median ridge on 

 the carapace, but, in accordance with the suggestion afforded by 

 the structure of M e s o t h y r a, the English authors have demon- 

 strated that this median ridge is in reality a detachable median 

 plate which usually overlaps t)ie dorsal edges of the carapace 

 valves. As to the nature of the contact in front of this median 

 plate beginning at the nuchal furrow, the form of the rostral plate, 

 the structure of the eye node, whieh in Mesothyra bears a 

 pitlike depression at its summit, we are still left in uncertainty. 

 It is however deducible from the profuse illustration of the genus 

 by Jones -and Woodward that genetically, Rhinocaris 

 and Mesothyra stand to D i t h y r o c a r i s in the rela- 



U5tb an. rep't N. Y. state geol. 1898. p. 69, figures. 



