OLENELLUS AND OTHER GENERA OF MESON ACID^ 313 



The hypostoma may be almost globose and oval in outline, with 

 smooth posterior and postero-lateral margins [pi. 38, figs. 2 and 3], 

 or elongate oval with the margins more or less denticulated. 



Development of the dorsal shield. — From the observations made in 

 the description of Pcedenmias transitans [p. 307] it is concluded that 

 OlcncUits, as now restricted is: 



First. — A Holmia without large third segment or telson [pi. 32, 

 figs. 1-3]. Holmia stage. 



Second. — A form with large third segment but without a dorsal 



spine on the fifteenth segment [pi. 32, figs. 4-7]. Intermediate stage. 



Third. — A P<zdeu}nias with large third segment, large spine on 



fifteenth segment, and with rudimentary segments and plate-like 



pygidium posterior to the fifteenth segment [pi. 33]. 



Fourth. — A true Olenelhis with large third segment, fifteenth seg- 

 ment a long telson, and without observable segments or plate-like 

 pygidium posterior to the fifteenth segment [pi. 34, fig. 9; pi. 35, 

 fig. i]. Olenelhis stage. 



The Nevadia stage of Olenellus is unknown, unless it is repre- 

 sented by figs. 2 and 3, pi. 32, where the pleurse of the thoracic 

 segments are apparently simple and unfurrowed. It is quite probable, 

 however, that the Nevadia stage has, by acceleration, been passed 

 and lost in the development of Olenellus. 



The telson of Olenelhis has long attracted the attention of paleon- 

 tologists. Prof. R. P. Whitfield said of it in 1884 [p. 152] : 



" This feature of the pygidium is so distinctive among all other 

 trilobites that it alone would serve as a generic distinction, and if the 

 condensation of parts indicates development of organization this form 

 would appear to be below even Paradoxides and should precede it 

 in age." 



In commenting on Whitfield's observations in 1886 I said [Wal- 

 cott, 1886, p. 166] : 



" From our present knowledge of these forms we reverse the 

 application made above and regard the telson as representing the 

 condensed parts, and the form as higher in organization and suc- 

 ceeding Paradoxides in time." 



Dr. B. N. Peach [1894, p. 672] considered the telson of Olenellus 

 as the homologue of the small pygidium of Holmia and Mesonacis. 

 Dr. John E. Marr [1896, p. 764] wrote: " The posterior segments 

 of the remarkable trilobite Mesonacis vermontana are of a much 

 more delicate character than the anterior ones, and the resemblance 

 of the spine on the fifteenth ' body-segment ' of this species to the 



