Genus Agnostits. — 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- 

 emtus, from Salem, New York. This species is illustrated in the 10th 

 Report U. S. Geol. Survey, p. 630, pi. 80, fig. 5. 



RECAPITULATION * 



From the Olenellus zone, Agnostus nobilis Ford, A. desider- 



atus Walcott, A. fallax Liurs 3 



From the Paradoxides zone 44 



From the Olenus zone 6 



From the Dikelocephalus zone 19 



From the Asaphus zone 18 



~9U 

 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. pisiforniis, plate ix, fig. 14), 

 In the section Limbati, the glabella is broadly rounded in front, 

 showing a great development as in ^4. 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. gihJnis. 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. 



