220 TRANSACTIONS OF THE [May 18, 
The pygidium is more remarkable. One of ? mm. has an oval 
form; the anterior margin is straight and without fold, mar- 
ginal fold around the rest of the shield widening backward. 
Rachis oblanceolate, trilobed by faintly impressed furrows, 
front lobe narrow, middle lobe wide but not tuberculate, side 
lobes with two transverse furrows, the posterior about one-third 
from the front. 
In this individual I conjecture that the anterior of the two 
furrows on the side lobes, will become the anterior marginal fur- 
row of the pygidium, and the posterior a first furrow on the 
side lobe; the pygidium would then agree with a similar stage of 
Microdiscus as regards these lobes, but not as regards the rachis. 
This minute pygidium shows a simpler structure than the 
adult, which, notwithstanding that the rachis is not segmented, 
is sometimes so preserved as to show the number of somites in 
the rachis. Thus the adult caudal shield (see pl. xv., fig. 11d) 
is sometimes well enough preserved to show on the cast of the 
inner surface a row of minute tubercles on each side of the 
rachis (five in number) and two on the axial line; the lateral 
tubercles are placed on the line between the somites, and each 
of the axial tubercles in the middle of a somite; it thus appears 
that the somites are in pairs, one with and the other without an 
axial tubercle. The first and second somites correspond to the 
first and second lobes of a Longifront pygidium, and the re- 
mainder of the axis to the third lobe in the pygidium of an in- 
dividual of that section of Agnostus. Also we observe that 
while in the first larval stage there are only three somites recog- 
nizable in the pygidium, the adult caudal shield has six. 
Agnostus declivis may be compared with the later A. secretus 
Walc., of the Prospect Mountain group, which it resembles in 
general form as well as in having the glabella tumid behind. 
Neither this variety nor the type of A. Acadicus appears to 
have any very near allies among the species of the Paradoxides 
beds of Sweden, known to the author. 
Size—Length, about 7 mm. Width,3 mm. Cephalic shield 
8x35 mm. 
Horizon and Locality.—A bundant in the fine, dark shales of 
Div. 1 d! at Porter’s Brook, St. Martin’s. 
AGNOSTUS PARVIFRONS Linnarsson. 
Agnostus parvifrons Linrs. Om. Vestergotland Cambr. 0. 
Silur. aflagr., p. 82, tafl. 2, fig. 56, 57. 
Agnostus parvifrons Tullb. Om. Agnostus-arterna, p. 34, 
tafl. ii., figs. 26, 27, 28. 
