\ 



244 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 53 



OlcncUns lapn'orthi [pi. 39, fig. 7], and Olcnelhis claytoni [pi. 40, 



fig-- 9]- 



The absorption of the spines and the resultant smooth margin 

 appears to have been accomplished in Callaiia broggeri [pi. 27, fig. 

 2], C. crosbyi [pi. 28, fig. 6], OlcncUns canadensis [pi. 38, figs. 2 

 and 3], Holmia lundgrcni [pi. 40,. fi-g. 6], and Mcsonacis torrcUi 

 [pi. 26, figs. 9 and 10]. 



The hypostoma of OlcncUns has the macuU^ clearly indicated, but 

 none of the specimens are sufficiently well preserved to permit of 

 making thin sections to determine its structure. 



Thorax. — As shown by adult specimens, the development of the 

 thorax from Ncvadia to OlcncUns, inclusive, may be divided into 

 six stages. 



1. Nc-vadia stage: In Ncvadia the thoracic segments of a uniform 

 character follow each other from the first to the seventeenth. At 

 the eighteenth segment an abrupt change occurs [pi. 23, figs, i, 2, 

 and 4]. The grooved pleural lobe disappears and a spinose exten- 

 sion of the same general character as that attached to the anterior 

 pleural lobes is attached directly to the side of the axial lobe of the 

 posterior eleven segments. The dorsal shield is terminated by a very 

 small and simple pygidium. 



2. Mcsonacis stage: In Mcsonacis [pi. 26, fig. i] the thoracic 

 segments are fully developed from the first to the fourteenth. The 

 third segment is enlarged and the fifteenth segment has a large 

 median spine and the ten posterior segments form a distinct sub- 

 ordinate series of small but typical segments. The smaller posterior 

 segments are more advanced in development than the posterior seg- 

 ments of Ncvadia, but not as much so as the anterior segments an- 

 terior to the fifteenth segment. 



3. EUiptoccphala stage: In EUiptoccpliala the third segment is 

 relatively larger during the earlier stages of growth in which it 

 has been observed [pi. 24, figs. 3-5], but this disappears in the adult 

 [pi. 24, fig. i], leaving the segments of a uniform character back 

 to the fourteenth where there is a series of five short segments with 

 long median spines. Most of the series of small segments of Ncvadia 

 and Mcsonacis have disappeared. 



4. Holmia stage: In Holmia the 16 segments are in orderly suc- 

 cession and of a similar character ; the pygidium remains relatively 

 small and more or less rudimentary. This is best shown by Holmia 

 kjcriilii [pi. 27, fig. 7] and H. roivci [pi. 29, figs. 3 and 4]. In 

 IVanncria za'alcottanns a short, slender spine appears on the four- 



