248 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



Wanneria. — Wanneria [pi. 30] has a uniform series of thoracic 

 segments with the pleurae terminating in broad falcate extensions 

 beyond the body line [pi. 30, fig. i] instead of spinose ends as in 

 Holmia [pi. 27, fig. 7]. It has a great median spine on the fifteenth 

 segment, much like that of Mesonacis [pi. 26, fig. i] and Pcedeumias 

 transifans [pi. 33]. The posterior segments are not rudimentary as 

 in the latter species. For comparison with Callavia see above. 



The species referred to Wanneria are: W. walcottanus (Wanner), 

 W. gracile Walcott, and JV. halli Walcott. 



Padcumias. — The posterior rudimentary thoracic segments of P<t- 

 deumias transitans [pi. 33] appear to be the result of the absorption 

 of the posterior segments of a many segmented ancestor and are 

 unlike the rudimentary posterior segments of Nevadia, which I 

 think are the originally less developed segments and which re- 

 cord a stage in the early evolution of the Mesonacidas that has not 

 been found in any other known species. The pygidium is also rudi- 

 mentary. The distinctive characters of the genus are in the presence 

 of the rudimentary segments and pygidium. 



The only species referred to Pcedeumias is P. transitans Walcott. 



Olenellus. — That Olenellus [pis. 37-39] should result from the 

 great development of the median spine on the fifteenth segment and 

 the absorption of the posterior rudimentary segments and pygidium 

 of its Mesonacis stage of development [pi. 33] is of great interest, 

 as it proves it to be the last phase of development of this line of 

 the Mesonacid^e. Olenellus thoinpsoni passes through a Holniia 

 [pi. 32, figs. 1-3] and Pcedeumias stage [pi. 33] before becoming a 

 true Olenellus. 



The American species referred to Olenellus are: 0. thonipsoni 

 Hall and its variety erassiniarginatus Walcott, 0. gilberti Meek and 

 an undetermined variety. 0. frenionti Walcott. 0. canadensis Wal- 

 cott. O. elaytoni Walcott, 0. argentus Walcott, and 0. zcalcotti 

 (Shaler and Foerste). 



The European species are: 0. gigas Peach, 0. lapzvorthi Peach, 

 O. retienlatus Peach, and Olenellus 2 sp. undt. 



Peachella. — Peachella [pi. 40, figs. 17-19] is known only by its 

 cephalon. This indicates a type related to Wanneria gracile [pi. 38, 

 figs. 21 and 22] or the younger stages of growth of Olenellus 

 canadensis [pi. 38, fig. 6]. It is probable that the thorax and py- 

 gidium will be found to be much like that of Olenellus. 



The only species known is P. iddingsi (Walcott). 



Olenelloides. — Olcnelloides is clearly defined by its large cephalon 

 and primitive thorax and pygidium. 



