28 



Fossils of the Utica Slate. 



sides are <almost parallel, the glabella furrows have become oblique 

 to the axis of the glabella and assumed the slight curvature that 

 appears in the adult. The cheeks have not developed as rapidly as 

 the glabella being now proportionally narrower. The eyes and free 

 cheeks also appear to have been retarded in a measure, but their 

 small size, and the fact that they are almost invariably broken away 

 or pressed under the head renders a study of them very difficult. 



The proportions of the head, thorax and pygidium vary proportion- 

 ately up to the adult. Each deep dorsal groove extends the entire 

 length of the body with a gentle curvature near the pygidium and 

 uniting at the posterior extremity of the axis of the pygidium. 



The pygidium retains more of the characters of the first degree 

 than either the head or thorax. 



Seve7ith to Thirteenth Degrees. Plate ii, figures 7-12. — 

 The development of the thorax continues by the addition of 

 segments, while all parts increase in size, with the development 

 of ihe individual. The strong convexity of the earlier stages is re- 

 duced, and with it, the very deep dorsal furrows. The entire ex- 

 pression assumes that of the adult. The examples of the r2th 

 degree are uniform with the smaller individuals of the 13th and 14th 

 from the same locality. 



Tv^elfth Degree to complete development in size. Plate ii, figures 

 13 and 14. — With the exception of the reference made by Pro- 

 fessor J. Hall to the presence of fourteen to fifteen segments in a 

 single individual and Mr. Billings to fifteen, Triarthrus Heeki has 

 been described as having thirteen segments in the thorax, this must 

 have been the usual number of segments met with by authors who 

 have had the species under consideration. This result has not been 

 obtained in our researches. Of forty-one examples, having respec- 

 tively thirteen, fourteen, fifteen and sixteen segments in the thorax, 

 six have thirteen, twenty-five have fourteen, seven have fifteen and 

 three show sixteen. 



The fully developed adult of sixteen segments varies but little, 

 with the exception of size, from the smaller individual of thirteen 

 segments. The larger adults are usually flattened by compression, 

 while the smaller individuals preserve the natural convexity of the 

 shell ; but when the conditions of preservation are considered the 

 larger are less convex when in a natural condition as is shown by the 

 transverse flattening of the segments in the axis of the thorax and 

 the greater expansion of the pygidium when the evidence of compres- 

 sion is very slight. Upon the glabella we find in the larger speci- 



