Genus Agnostus.— Vogdes. 381 
1889—Walcott describes in the Proc. Natl. Mus. vol. 12, 1889, the only 
known American Olenellus zone Agnostus, under the name of A. desid- 
eratus, from Salem, New York. This species is illustrated in the 10th 
Report U. 8. Geol. Survey, p. 630, pl. 80, fig. 5. 
RECAPITULATION.* 
From the Olenellus zone, Agnostus nobilis Ford, A, desider- 
atus Walcott, A. fallax Lins 22.0: Seo Ooo 3 
Peale. Paradoxes ZONGC 56. .cncarnect esaceeesseee-- 44 
From the Olenus’ zone................. apie hiease cleo (6 
rom tic. Dikelocephalus: 20nC.. 0... .c eects cece es cece: 19 
RULE VER ASA DINUS! (ZONE. sa ag sme s-ceisle celelaelae ee sess «+ = 18 
90 
AGNOSTUS, Brongniart, 1822. 
Diagnosis.—The general form of the body of the Agnosti is 
elongated elliptical, the surface convex. The head presents the 
same structure as that part in other trilobites, with the exception 
of eyes and facial suture, which are wanting in this genus. The 
glabella determined by the dorsal grooves, never extends to the 
anterior contour; it is always prominent by its relief; its form 
varies with the species. The typical form has a glabella divided 
into a small frontal lobe, and a larger posterior lobe; the basal 
lobes forming a third part. The frontal lobe is usually subtrian- 
gular having a groove in front (A. pisiformis, plate 1x, fig. 14). 
In the section Limbati, the glabella is broadly rounded in front, 
showing a great development as in A. rex, plate x, fig. 13. In 
the section Parvifrontes, the glabella has only a single lobe, as in 
A, parvifrons, plate x, fig. 12. In the typical glabella the pos- 
terior lobe exhibits an inclination to divide laterally, or in front, 
being marked by a medium ridge. The glabella is sometimes 
compressed on the sides by the basal lobes, but expands at this 
point again in its posterior projection, as in A. gibbus. The 
basal lobes are very short, so that in A. gibbus, they appear like 
a narrow band widening out on both sides of the head, forming 
two nodes, which are sometimes large and triangular, as in the 
section Fallaces. The basal lobes are divided in some species 
into two nodes on each side, as in A. atavus, but, they are gener- 
ally very minute. The occipital groove and ring are more or less 
developed. The lateral lobes of the head form a concentric band 
with the contour; this is called by Barrande the genal zone. In 
the section Longifrontes, the cheeks in front of the glabella are 
divided by a furrow extending from the apex of the glabella to 
*The varieties are omitted in this enumeration. 
