OLENELLUS AND OTHER GENERA OF MESONACID^ 245 



teenth segment in fully matured dorsal shields [pi. 30, figs. 10-12] ; 

 otherwise the segments are of the same character, except as they 

 decrease uniformly in size to the pygidium. In Callavia brog- 

 gcri [pi. 27, fig. i] the seventeenth and eighteenth segments are rela- 

 tively smaller, in this respect resembling the shorter posterior seg- 

 ments of Mcsonacis and FJliptocephala. 



5. Pccdeumias stage: In Pcudeumias transitans [pi. 33] we find 

 the transition stage between the stages represented by Mesonacis 

 [pi. 26], Elliptocephala [pi. 24], and Olenellus thompsoni [pi. 35]. 

 There are fourteen fully developed segments with the third segment 

 enlarged and the fifteenth segment developed into a strong, long 

 spine with the pleural lobes of the segment absent. Beneath and 

 back of the large spine there are from two to six rudimentary seg- 

 ments without pleural lobes, and a simple plate-like pygidium. 



6. Olenellus stage: Fourteen fully developed segments, a large 

 third segment, and the fifteenth segment a strong terminal telson ; 

 posterior rudimentary segments and true pygidium of the Pccdeumias 

 stage absorbed [pi. 35, fig. i] or the rudimentary segments and 

 pygidium have disappeared and the large median spine of Pccdeumias 

 has become the telson of Olenellus. 



Olenellus is the last genus of the Olenellus branch of the Mesona- 

 cidae to develop, and from Pcedeuniias transitans we find evidence 

 that it has passed through the Holmia stage [pi. 32, figs. 2 and 3] 

 and the Fardcuinias stage [pi. 33] before becoming a true Olenellus. 



The enlarged third segment of Olenellus [pi. 35, fig. i] also occurs 

 in Mesonacis [pi. 26, fig. i] and in the younger stages of growth of 

 Elliptocephala [pi. 24, figs. 3-5]. In Olenelloidcs [pi. 40, fig. 3] 

 both the third and sixth segments are enlarged. The cause of the 

 enlargement and prolongation of the third segment is unknown. In 

 Paradoxides the pleurae of the second segment are elongated in small 

 specimens of several species [Barrande, 1852, pi. 10, fig. 25; pi. 12, 

 figs. 5-7; and pi. 13, fig. 16]. 



Peachella. — We know nothing of the thorax of Peachella [pi. 40, 

 figs. 17-19], but from the cephalon it. is probable that it was of the 

 Olenellus type. 



Olenelloidcs. — The thorax and large cephalon of Olenelloides [pi. 

 40, fig. 3] indicates a degenerate type and a stage of growth beyond 

 which the animal could not develop. For the present it may be 

 placed as a degenerate of the Olenellus stage of development. 



Pygidium. — The pygidium is a simple transverse plate in the 

 protaspis stage and it is not strongly developed in any genus and 



