26 



Fossils of the Utica Slate, 



of the 1st order, will be discovered for Triarthrus Becki is extremely 

 doubtful. 



Degrees of Development. 



First Uegree. Plate ii, figures 1, la and 15. — Of the four 

 individuals indicating the first degree of development, as known at 

 the present time, three have each a total length of one and one-eighth 

 millimetres, and one of one millimetre the latter being proportion- 

 ally broader than the former. The anterior and posterior extremi- 

 ties are broadly rounded, giving a general oval outline. The entire 

 individual is strongly convex, and the two deeply impressed dorsal 

 furrows give a marked convexity both to the central and lateral lobes. 



The head and pygidium are distinctly separated by one thoracic 

 segment and the trilobitic character is as essentially formed as in 

 the later degrees of development. The head is a little more than 

 one-half the entire length of the animal; the glabella is elongate, 

 narrower at the base than at the anterior margin, and the glabellar 

 furrows are indicated by straight transverse furrows penetrating 

 one-third the distance across the glabella and are proportionately 

 farther apart than in the fully developed glabella of the adult. 



Occipital groove, transverse, deeply impressed. Occipital ring, 

 convex, rising in the center to form the base of a small node, or 

 short spine ; transverse anteriorly, it slopes each way from the 

 center of the posterior margin to half the width at the lateral 

 margins. The dorsal grooves do not extend in a direct line with 

 the sides of the glabella, as in the adult, but turn obliquely out- 

 ward, across the occipital segment at the base of the glabella, 

 giving a constricted appearance to the central lobe of the head at 

 the occipital groove. Fixed cheeks convex, broad at the base, nearly 

 equaling the width of the posterior margin of the glabella; posterior 

 groove very clearly defined, extending across on a line with the 

 posterior margin of the occipital segment, curving slightly forward 

 near the lateral margin; the posterior margin is a rather narrow 

 elevated rim. The eyes and free cheeks are not discernible in any 

 of the examples of this degree ; it may be owing to the condition 

 of their preservation as it is often difficult to discover these parts in 

 the third degree of development. 



The thorax is shown by the presence of one segment. The pleurae 

 are grooved as in the later degrees of development and the charac- 

 teristic spine upon the center of the axis is distinctly shown. 



The pygidium is strongly lobed; the deep dorsal furrows giving 



