Fossils of the Utica Slate., 



27 



prominence to the strongly convex axis. The five rings crossing it 

 are prominent and sharply defined; the last is scarcely more than a 

 transverse node as the axis tapers to a point within the posterior 

 margin. The anterior ring of the lateral lobes shows a pleural 

 groove crossing it obliquely which separates a small lobe or elongate 

 margin between it and the thoracic segment, a feature which is 

 persistent teethe adult; the three posterior rings are simple rounded 

 pleurae curving slightly backward toward the margin; the repre- 

 sentative of the last ring of the axis is a broad expansion of the last 

 segment extending across the posterior margin. 



Second Degree. Plate ii, figure 2. — All parts have increased 

 in size but not proportionally. The table (p. 32), illustrates this 

 very completely, as it does for all the degrees of development. The 

 glabella has widened, and the occipital ring narrowed slightly at 

 the center. The free cheek is seen as a narrow rim. The eye is 

 indicated by a very small groove on the outer border of the fixed 

 cheek which in the later stages of growth separates the palpebral lobe 

 from the fixed cheek. 



The thoracic segments are convex; each is raised at the center to 

 form the base of a short spine. This spine is very prominent in all 

 well preserved specimens, from the first to the last degree of de- 

 velopment. The pleurae are very convex on each side of the pleural 

 groove; they terminate nearly the same as in the adult. 



Third, Fourth and Fifth Degrees. Plate ii, figures 3, 4 and 5. — 

 The details of the development in size may be seen by a glance at 

 the table. The spine on the axis of each thoracic segment appears 

 with it, as it must from the fact that we find the spine on the axis 

 of the first anterior segment of the pygidium, and frequently on the 

 second when the pygidium has six- rings in the axis, as it does in in- 

 dividuals having from three to seven segments in the thorax. With 

 a sufiicient amount of material we would probably find this to hold 

 good in all the degrees of development as was observed by M. 

 Barrande, the thoracic segments were first elaborated in the pygi- 

 dium before being liberated in the thorax. In one very perfect 

 example, with five segments in the axis of the thorax and six in the 

 pygidium, the first three anterior segments of the pygidium have 

 the spine upon them. 



Sixth Degree. Plate ii, figure 6. — At this stage all the parts 

 have attained most of the characters of the adult. 

 The glabella has gradually expanded towards the base so that its 



