Part 3

Genus Fourth. Paradoxides.

Body depressed not contractile.

Flanks much broader than the middle lobe.

Buckler nearly semicircular three transverse furrows on the middle lobe.

Eye-shaped tubercles none.

Abdomen with 12 articulations.

Arches of the lateral lobes, more or less prolonged beyond the membrane which sustains them.

Genus Fifth. Agnostus.

Body ellipsoidal--semicylindrical.

Buckler and flanks edged--the edges being slightly elevated.

Middle lobe with two transverse divisions, each composed of a single piece.

Two glandular tubercles on the anterior part of the body.

In 1824, Dr. J. E. Dekay added a sixth genus to the family of the trilobites, which he describes in the following manner.

Genus Sixth. Isotelus.

Body oval oblong, often contracted, not unfrequently extended.

Head or buckler large and rounded, equalling the tail in size, with but two oculiform tubercles.

Abdomen with 8 articulations.

Frontal process beneath, with two semilunar terminations.

Post-abdomen or tail broad, expanded with indistinct divisions, as large as the buckler.

Longitudinal lobes very distinct.

This genus, he remarks, will be sufficiently distinguished from the five genera proposed by M. Alexandre Brongniart in his valuable and truly philosophical work on the trilobites by the following particulars.

From Calymene. By the presence of but two tubercles on the buckler not reticulated; by the abdomen with but 8 articulations.

From Asaphus. By the middle lobe, which is double the size of the lateral ones; by the absence of a membranaceous expansion on the sides; by the non-reticulation of the eyes, &c.

From Ogygia. By the rolled form, the rounded posterior angles of the buckler, and the distinct articulation of the longitudinal lobes.

From Paradoxide and Agnoste by characters too obvious to be enumerated. (See Annals of N. York Lyceum, Sec. Vol. I. pp. 174-5.)

In 1826, J. W. Dalman published in the Transactions of the Swedish Academy, and also in a separate work, an account of the trilobites found in the North of Europe, in which he has enriched the family by a number of fine species, and with the following genera, which he modestly proposes merely as subdivisions.

Genus Seventh. Nileus.

Body short, capable of contraction into a sphere, smooth, convex.