Genus Agnostus.— Vogdes. 383 
an elevated ridge extends from the spines of the pygidium to the 
axis. 
In the section Levigati, the dorsal grooves are limited on the 
head, only partly defining the glabella; they are also limited on 
the pygidium, especially in rear, as in A. /evigatus Dalm., plate 
X, fig 3. In the section Arthrorhachis the glabella is prominent, 
long and marked with small basal lobes; the axis of the pygidium 
is short, as in Agnostus tardus Barr. 
PART 3. DESCRIPTION OF THE NORTH AMERICAN SPECIES. 
The insignificant genus Agnostus surpasses in number of spe- 
cies all the other genera of the order Trilobita; in the primordial 
fauna we have a total number of ninety species, excluding the 
varieties. The species range from the Olenellus zone to the 
Lower Silurian. The zone with Paradoxides contains 44 species 
and several varieties; the Olenus is not so well represented; but, 
the genus outlived both these genera and extends into the Cera- 
topyge chalk (A. sidendladhi) also in the Orthoceras zone 
(A. lentiformis &c). The genus dies out in the zone with 
Asaphus and Trinucleus (A. trinodus &c). In North America 
the genus is represented by twenty-eight species which may be 
arranged into the following zones and sections: 
OLENELLUS ZONE. 
AGNOSTUS NOBILIS Ford, 1872. 
This is a doubtful species and may belong to the genus Micro- 
discus. 
Section |, PARVIFRONTES.—The glabella is only partly de- 
veloped in this section (Agnostus brevifrons Ang., plate x, fig. 12). 
AGNOSTUS DESIDERATUS Walcott, 1890. 
Plate x, fig. 7. Cf. Agnostus prolongus Hall. 
Diagnosis, Head about as broad as long, broadly rounded in 
front, sides curving in slightly towards the posterior margin, 
which slopes obliquely inwards from the postero-lateral angles to 
the glabella. The glabella is less than } the length of the 
head. <A narrow raised rim extends all around the margins, ex- 
cept across the base of the glabella, which is subeylindrical, nar- 
row, with a small node on the posterior third of its medium line. 
Surface smooth. Locality, Salem, N. Y. This species is of the 
