Genus Agn ostics. — Vogdes. 383 



an elevated ridge extends from the spines of the pygidiiim to the 

 axis. 



In the section Lcevigati^ 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. Icevigatus 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 (.4. sideadladhi) also in the Orthoceras zone 

 {A. lentiformis &c). The genus dies out in the zone with 

 Asaphus and Trinucleus (^4. 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 I, PARVIFRONTES.— The glabella is only partly de- 

 veloped in this section {Agnostus hrevi/rons Aug., plate X, fig. 12). 



Agnostus desideratus Walcott, 1890. 

 Plate X, fig. 7. Cf. Agnostus jn-olongus Hall. 

 Diagnosis. Head about as broad as long, broadl}' rounded in 

 front, sides curving in slightly towards the posterior margin, 

 which slopes obliquely inwards from the posterolateral angles to 

 the glabella. The glabella is less than f the length of the 

 head. A narrow raised rim extends all around the margins, ex- 

 cept across the base of the glabella, which is subcylindrical, 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 



