The Trilobites have been arranged in numerous genera, the names of which in a few cases are expressive of natural characters, but in others have reference to the obscurity that still invests some parts of the organization of these animals.[482]
[482] As, for example, Asaphus, obscure; Calymene, concealed; Agnostus, unknown.
Calymene Blumenbachii. [Lign. 175], figs. 3, 4.—This is the Trilobite so well known as the Dudley locust, or insect. It consists of an ovate, convex, trilobed crustaceous shell, or case, and is found either expanded, as in [Lign. 175], fig. 3, with its under surface attached to, and blended with, the limestone (Wond. p. 789); or coiled up like an Oniscus, or wood-louse, as in figs. 4 and 4a. The head is large, convex, rounded in front, with a broad border, and divided into three lobes by two longitudinal depressions. The eyes are two in number, compound, and have numerous facets; they are situated on the sides of the head, remote from each other. The carapace is deeply trilobed by two longitudinal furrows; the thoracic portion is composed of thirteen segments; the caudal shield is small and nearly semicircular. This species is from one to four inches in length. It occurs from the Lower Llandeilo rocks up to the Upper Ludlow inclusive.
The structure here described may be regarded as the normal type, but numerous and important modifications prevail in the different genera.
Lign. 176 Homalonotus dephinocephalus.
(Reduced from pl. vii. Sil. Syst.) Upper Silurian. Dudley.
In the genus Homalonotus, [Lign. 176], the thoracic portion of the carapace is but obscurely lobed, and consists of thirteen segments; the abdominal is distinct from the thoracic, and formed of nine rings; it terminates in a prolonged point. The H. Herschelii is a large Trilobite, very plentiful in the Upper Silurian schists of the Cape of Good Hope.
In another genus, Asaphus (Geol. Surv. Decade 2), the carapace is wide and much depressed; the middle lobe distinct, the cephalic portion rounded in front, and terminating posteriorly in a sharp process on each side. The eyes are compound, and each contains upward of six thousand lenses, many of which remain in some examples.[483] Some American species belonging to this group are of a gigantic size, as, for example, the Isotelus gigas (of Mr. Dekay), which is eighteen inches long. In the Isotelus[484] the body is of an oval shape, and the posterior angles of the head are rounded; the thorax is composed of eight segments.