1896. ] NEW YORK ACADEMY OF SCIENCES. 217 
A head shield associated with this pygidium does not differ 
from the type of A. fallax, but the pygidium does. 
Pygidium quadrate, somewhat wider than long, marginal fold 
and furrow sharply defined in front, wide at the sides and with 
strong spines. Rachis trilobed by two strong transverse furrows, 
which are deeply cut in the middle and also at the extremities 
with a shallower space between ; the anterior lobe is narrow with 
lateral tubercles, middle lobe crossed by a strong keel-like 
tubercle, posterior lobe having a somite about as wide as the 
middle lobe, marked off at the front by a shallow furrow, not 
always distinguishable. 
Size.—Length of the caudal shield, about 3 mm. 
Horizon and Locality.—Fine grey shales of Div. 1 ¢.1. St. 
John. Scarce. 
This pygidium is of considerable interest because it represents 
in the adult condition a larval phase of Agnostus. It is instruc- 
tive to compare it with the 1 mm. larval form of A. fallax (type) 
(pl. xv., fig. 8b) which shows a similarly, but more imperfectly 
. segmented rachis. This pygidium is the only one of the Fallax 
section which is distinctly segmented ; it thus expresses a con- 
dition of that shield quite common in the Longifrontes, and 
partly for that reason was at first thought to be the caudal 
shield of A. partitus; the general facies however, is that of a 
Fallax. 
AGnostus Acapicus Hartt. pl. xv., figs. 10a and b. 
Agnostus Acadicus Hartt. Acad. Geol. 2d ed. p. 665, fig. 229, 
U.S. Geol. Surv. Bull. 10 p. 22, pl. ii., figs. 2a, band c. Trans. 
Roy. Soc. Can. vol. iii., p. 70, pl. vii , figs. 5@ and b. 
Head minute, transversely elliptical, or subcirecular, breadth 
and length about equal, convex but very depressed, outlines in 
front and on the sides slightly straightened. A narrow, flat- 
tened and but very slightly elevated border goes round the 
front and lateral margins. This is separated from the rest of 
the shield by a narrow, shallow, flat space, or groove, which on 
going posteriorly along the lateral margins loses gradually in 
width towards the posterior angles of the shield, which are 
rounded. Glabella a little less than two-thirds of the length of 
the shield, long elliptical, depressed convex, but more elevated 
than other parts of the shield, about twice as long as broad, 
bounded anteriorly and laterally by a sharp but rather deep 
groove concentric to the outer one above described. A well 
marked transverse furrow arching backward separates the an- 
terior third of the glabella as a subcircularlobe. Posterior part 
of the glabella rounded, but impressed on each side by a little 
