Fig. [136].—Homalonotus delphinocephalus, Green, × 1. Silurian. (After Zittel.)
In most cases the only part of the Trilobite which is preserved is the exoskeleton which covered the dorsal surface of the body. That skeleton consists largely of calcareous material, and shows in sections a finely perforated structure. Generally it is arched above, but in some cases is only slightly convex; in outline it is more or less oval. Three regions can always be distinguished in the body of a Trilobite—the head, the thorax, and the abdomen or pygidium.
The carapace which covers the head is known as the cephalic shield (Fig. [137], A, 1), and is commonly more or less semicircular in outline, but varies considerably in different genera. Only in a few cases, as in some species of Agnostus (Fig. [146]), is its length greater than its breadth. The axial part of the cephalic shield, called the “glabella” (Fig. [137], A, a), is usually more convex than the lateral parts (“cheeks” or “genae”), and is separated from them by longitudinal or axial furrows (b). The shape of the glabella varies greatly; it may be oblong, circular, semi-cylindrical, pyriform, spherical, etc. Its relative size likewise varies; thus in Phacops cephalotes it expands in front and forms the larger part of the head, whilst in Arethusina (Fig. [151], B) it is narrow and short, being only about one-half of the length of the head.
Fig. [137].—Calymene tuberculata, Brünn. × 1. Silurian, Dudley. A, Dorsal surface: 1, head; 2, thorax; 3, pygidium or abdomen. a, Glabella; b, axial furrow; c, glabella-furrow; d, neck-furrow; e, fixed cheek; f, free cheek; g, facial suture; h, eye; i, genal angle; k, axis of thorax; l, pleura. B, Ventral surface of head (after Barrande): a, hypostome; b, doublure; c, c′, facial sutures; d, rostral suture; e, rostral plate. C, One segment of the thorax: a, ring of axis; b, groove; c, articular portion; d, axial furrow; d-f, pleura; d-e, internal part of pleura; e-f, external part of pleura; e, fulcrum; g, groove. D, Coiled specimen: a, glabella; b, eye; c, facial suture; d, pygidium; e, rostral suture; f, continuation of facial suture.
The segmentation of the head is indicated by transverse furrows on the glabella (Fig. [137], A, c, d). In some cases these furrows extend quite across the glabella (Fig. [147]), but commonly they are found on the sides only and divide the glabella into lateral lobes. Only the posterior or “neck-furrow” (Fig. [137], A, d) is continued on to the cheeks, and the segment which it limits anteriorly on the glabella[[180]] is known as the occipital or neck-ring. In front of the neck-furrow there may be three other furrows, so that altogether five cephalic segments are indicated by the furrows of the glabella. Commonly all the furrows are distinct in the primitive types; but in the more modified forms some, especially the anterior, become either reduced in size or obsolete. The actual number of furrows present consequently varies in different genera, and may even differ in different species of the same genus. In a few genera all the furrows are either indistinct or absent, as for example in Ellipsocephalus (Fig. [150], B). In some cases four furrows are present in addition to the neck-furrow; this is due to the division of the anterior lobe of the glabella by fulcra which are developed for the attachment of muscles.
When the glabella reaches the front border of the head the two cheeks are separated (Fig. [150], I); but in other cases they unite in front of the glabella (Fig. [150], C). The outer posterior angle of the cheeks or genae (“genal angle,” Fig. [137], A, i) may be rounded, pointed, or produced into backwardly directed spines (Fig. [140]). The marginal part of the cephalic shield is often flattened or concave; this border may be quite a narrow rim as in Calymene (Fig. [137], A), but in some genera (e.g. Trinucleus, Fig. [140], B; Harpes, Fig. [150], A; Asaphus) it attains a great development. Each cheek is usually divided by a suture—the “facial suture” (Fig. [137], A, g)—into an inner and an outer part; the former is the “fixed cheek” (e), and the latter the “free cheek” (f). The course of the facial suture varies in different genera: on the posterior part of the head it begins either at the posterior margin (Fig. [150], C) or at the posterior part of the lateral margin (Fig. [151], C, D); at first it is directed inwards, and then bends forward, forming an angle. In front it may (a) end at the front margin (Fig. [147]), or (b) be united beneath the front margin by a rostral suture (Fig. [137], B, d, D, e), or (c) unite with the other suture on the dorsal surface in front of the glabella (Fig. [151], C). In the last case the free cheeks also unite in front of the glabella.
The facial suture is one of the distinguishing features of the Trilobites, and may have been of some use in ecdysis. In only a few forms is it absent, as for example in Agnostus (Fig. [146]) and Microdiscus. In the former, however, Beecher states that a suture is really present, but, unlike that of most other Trilobites, it is situated at the margin of the cephalic shield, and consequently the free cheek, if present, must be on the ventral surface. Lindström and Holm, after a re-examination of well-preserved specimens, deny the existence of a suture in Agnostus. By most authors Olenellus is said to be without a suture, but Beecher maintains that although the fixed and free cheeks have coalesced, yet a raised line passing from the eye-lobe to the posterior margin marks the position of the suture; this view is not accepted by Lindström.
The existence of a facial suture in Trinucleus has likewise been disputed. But Emmerich, Salter, and M‘Coy[[181]] have maintained that a suture is present in a normal position on the dorsal surface, extending from the posterior margin just within the genal angle to the eye (when present), and from thence bending forward and ending on the front margin near the glabella. It must be admitted that no indications of the suture are seen in the majority of specimens, perhaps owing to the fact that most examples of Trinucleus are in the form of internal casts; perhaps also to the more or less complete coalescence of the fixed and free cheeks, since in no specimen has the free cheek been found separated from the rest of the head, as occurs not uncommonly in many other Trilobites. The probability of the existence of a suture receives some support from the fact that one is found in the allied genera Orometopus and Ampyx (Fig. [140]). Barrande and Oehlert deny its existence in Trinucleus. There is, however, in that genus a suture running close to the margin of the cephalic border,[[182]] and joining the genal angle so as to cut off the genal spine. Lovén and Oehlert claim that this suture represents the facial suture, but in an abnormal position; this view, however, is not accepted by Beyrich. In this connection it should be noted that in Acidaspis, whilst the majority of the species possess a facial suture, there are two in which it has disappeared owing to the fusion of the fixed and free cheeks. Such being the case, it seems not improbable that the curved line passing backwards from the eye in Harpes may mark the position of the suture; but it is stated that the only suture present in that form runs at the margin of the cephalic border, and is similar to that of Trinucleus. This matter will be referred to again when discussing the nature of the eyes in Trinucleus and Harpes.
The relative sizes of the fixed and free cheeks obviously depend on the position of the facial suture; when this starts on the lateral margin of the cephalic shield and passes forward to the outer part of the front margin, the free cheek will be a narrow strip; when, on the other hand, the suture starts from the posterior margin and runs close to the glabella, the free cheek will be relatively large and the fixed cheek narrow. The fixed cheek is small in Phacops, Cheirurus, and Illaenus; relatively large in Remopleurides, Phillipsia, and Stygina. It was suggested by M‘Coy[[183]] that the free cheek represents the pleura of an anterior segment which has not become fused with the other cephalic pleurae. The fixed cheek appears to be formed of the coalesced pleurae of the other cephalic segments, but of those pleurae the only indication seen in adult specimens is in the neck-ring; in young specimens of Olenellus, however, the presence of other pleurae is indicated by furrows on the cheeks in front of the neck-furrow.