OLENELLUS AND OTHER GENERA OF MESONACID-E 233 



INTRODUCTION 



This paper was first planned to include the descriptions of the new 

 genera and species of the Mesonacidae that had been collected by me 

 or under my direction since the publication of the memoir on the 

 Olenellus fauna in 1891 [Walcott, 1891]. When the material was 

 assembled, I wrote to my friend, Prof. Atreus Wanner, of York, 

 Pennsylvania, asking if he had any new material. In response he 

 sent me a beautiful series of specimens showing the growth of the 

 dorsal shield of Pccdcnmias transitans and specimens of Wanncria 

 walcottanus with a large spine on the fifteenth segment. I also found 

 in the collections from central Alabama a very interesting series of 

 specimens of the yotmg cephalons of Padcnmias and Wanncria. 

 The result has been that I have reviewed and discussed the family 

 Mesonacidse and illustrated the known genera and species more or 

 less fully. 



When in 1891 I proposed to use the term Mesonacidse [Walcott, 

 1891, p. 635] I thought it a better selection than to propose Olenel- 

 lid?e and so stated. Vogdes [1893, p. 254] evidently misunderstood 

 my intention and used the term Olenellidffi. Later Moberg [1899, 

 p. 316], evidently without knowing of Vogdes' use of the term, pro- 

 posed to use 01enellid?e, as he thought it did not conflict with Oleni- 

 dae. Lindstrom [1901, p. 12] simply followed Moberg. The term 

 Olenellidae is a good one, but ]\Iesonacidfe has priority, and also the 

 genus Mesonacis is much more typical of the family than the genus 

 OlcncUus; the latter is the last phase of the evolution of one branch 

 of the family, while Mesonacis illustrates the stage in which the 

 marked characteristics of most if not all of the genera are present. 



Mesonacis vermontana' (Hall) was founded on a specimen preserv- 

 ing the cephalon and a portion of the thorax [Hall, 1859, fig. 2, p. 60] . 

 In 1886 I found this form was essentially similar to Olenellus thomp- 

 soni [Walcott, 1886, pi. 24, fig. la] back to the fourteenth segment, 

 but that the fifteenth segment instead of being a telson was a thoracic 

 segment with a long median spine. Posterior to the fifteenth seg- 

 ment there were ten segments with short pleural lobes and a plate- 

 like pygidium [pi. 26, figs, i and 2]. For this strange form the 

 genus Mesonacis was proposed [Walcott, 1885, p: 328], and I [Wal- 

 cott, 1886, p. 166] concluded that the telson of Olenellus thompsoni 

 was represented in Mesonacis by the fifteenth segment and the pos- 

 terior segments and pygidium. Subsequently other specimens were 

 found with segments posterior to the fifteenth [pi. 26, fig. 3], and 

 one large specimen [see pi. 33, fig. i, and pi. 24, fig. 12] that had 



