56 The Ottawa Naturalist. [Aug.-Sept. 



they become fewer in number. Of known genera of the Meso- 

 nacidct, Mesonacis and Paedeumias have a spine-bearing fifteenth 

 segment, and the progression from the rib-like fifteenth seg- 

 ment of Mesonacis through the more rudimentary fifteenth 

 segment of Paedeumias robsonensis, and the almost telson-like 

 segment of Paedeumias transitans (which cuminates in the telson 

 of Olenellus) is paralleled by the progression from the rib-like 

 posterior segments of Mesonacis to the less rib-like segments 

 of Paedeumias transitans. Moreover, the close relationship of 

 the three genera is shown by the fact that in each the third 

 segment is enlarged. That the number of rudimentary seg- 

 ments alone bears little or no relation to the relative primitive- 

 ness of the form is indicated by the fact that Mesonacis, which 

 is clearly more primitive than Paedeumias, has less than one- 

 third the number of rudimentary segments. Nevadia, which 

 appears to be the most primitive as well as the earliest of the 

 Mesonacidae, does not seem to have reached the stage where 

 differentiation of its segments might take place. In it there 

 is a steady progressive decrease in the length of the pleural 

 groove from the first to the eighteenth, with from six to eleven 

 posterior segments whose pleural portion is unmarked. 



In Elliptocephala the five segments posterior to the anterior 

 thirteen (not fourteen as in the Mesonacis-Paedeumias-Olenellus 

 line) are all spine-bearing, and are identical in everything but 

 size. This feature has only been described for one other form, 

 namely, Redlichia chinensis, and while the posterior five seg- 

 ments in this species are spine-bearing and do not otherwise 

 differ from those anterior to them, we have no information as 

 to the number of the anterior segments. It is at least 12 (a), 

 however. In Wanneria there is a tendency toward nodes or 

 spines on the anterior thirteen segments, and the fourteenth 

 bears a short spine, but except in this respect it is indistin- 

 guishable from the progressively smaller segments posterior 

 to it. In this genus there is no suggestion of a resorption of 

 segments, and it seems natural to suppose that Holmia may 

 have been derived from it since that genus also betrays no 

 tendency toward resorption, and the anterior fourteen segments 

 only of the sixteen bear spines. In neither Holmia nor Wanneria 

 is there any enlargement of the third segment. 



The fact that there is no enlargement of the third segment 

 in Nevadia corroborates the indication given by the character 

 of its ribs, and appears to justify us in believing it to be very 

 primitive. The general resemblance between this genus and 

 species of Callavia such as eucharis and perfecta (b) is worthy 



(a) Walcott, Research in China, vol. 3, 1913, pi. 24, figs. 1, la. 



(6) Walcott, Smithsonian Misc. Coll., vol. 57, No. 11, 1913, pi. S3, figs. 1 and 3. 



