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.