OLENELLUS AND OTHER GENERA OF MESONACID.E 307 



and of 8 mm. to the fifteenth segment. This appears to have six 

 rudimentary segments and pygidium. The largest specimen from 

 York [fig. 10, pi. ^2] has a length of 74 mm. to the end of the 

 telson-like spine and of 47 mm. to the fifteenth segment. It has 

 five rudimentary segments and pygidium. The largest specimen 

 from Georgia, A'ermont, has a length of 98 mm. to the fifteenth seg- 

 ment. The entire dorsal shield of this specimen is similar to that 

 of OlciicUus thompsoni, except that the fifteenth segment is not 

 quite reduced to a telson, and three rudimentary segments and a 

 pygidium occur back of the great median spine [pi. 33, fig. i]. 

 Another feature to be noted is that the surface characters of the 

 rudimentary segments and pygidium are sharp, elevated subparallel 

 lines, as in the genus Paradoxidcs. and unlike those of the grcTt 

 spine and the segments of the thorax [pi. 24, fig. 12] which form a 

 network of irregular reticulating and inosculating elevated lines 

 characteristic of the known adult forms of most of the Mesonacid^. 

 Nearly all specimens of the thorax of P. fraiisitaiis have a sharp, 

 elongate, median node on the posterior four to six thoracic seg- 

 ments [fig. 10, pi. 32]. On some of the larger specimens there is a 

 slender, sharp node or spine at the posterior margin of the segments 

 from the first to the eighth, and back of the eighth the base of 

 the node or spine becomes more elongate until it extends across the 

 full width of the segment. The hypostoma has a denticulated or 

 spinous postero-lateral and posterior margin. The spines are short 

 and usually blunt [fig. 7, pi. 34], but they may be sharp [fig. 5] ; 

 there are five larger ones on each side and two or three smaller and 

 shorter ones on the back margin that are usually broken off or 

 obscure so as to give the effect of a clear space [fig. 5] without 

 spines. This type of hypostoma is quite abundant at the York 

 localities, and an almost similar form occurs in Alabama. 



Voiiiig stages of growth of dorsal shield. — The youngest stage of 

 growth collected by Professor Wanner is i mm. in length over the 

 cephalon [fig. i, pi. 1,2]. The next stage [pi. 32, fig. 2] is 1.5 mm. 

 in length with cephalon, five thoracic segments and a Hobnia py- 

 gidium. At this stage the thoracic segments do not show transverse 

 furrows on the pleural lobes and in such specimens [figs. 2 and 3] 

 the segments are rudimentary or have not reached the fully de- 

 veloped stages as seen in the adult [figs, i to 3, pi. 34]. This imma- 

 ture stage of the thoracic segment occurs in the posterior segments 

 of the adult form of Nevadia zceeksi [pi. 23, figs, i, 2, and 4], and 

 represents the earliest know'n or Nevadia stage of development of 



