Hinge-border. The line of junction of the two valves along which they remain united. Little teeth sometimes occur upon the hinge (fig. 124, c). Ventral border.-The lower border, towards which the limbs of the ostracod are directed during life. Eye-spot.-A fairly hemispherical tubercle occurring in some genera on the anterior part of the valve, and indicating the position of an eye. The Ostracods are mostly Marine; fresh or brackish water forms will be specially indicated.* Cypris (fig. 124, a).-Shell small, partly horny, and thin. Left valve the larger. Oval, or rather bean-shaped; ventral border commonly somewhat concave. No 00 α b C, Fig. 124.-a, Cypris purbeckensis (Lower Purbeck Beds). The left valve is towards the observer. b, Cypridea punctata, var: gibbosa (Middle Purbeck). Left valve. Cythere retirugata, var: rugu lata (Lower Purbeck). Interior of left valve, showing an anterior socket for the tooth of the right valve; then a tooth, followed by a barlike ridge, which terminates in a posterior socket for the posterior tooth of the right valve. (The three figures after Prof. Rupert Jones.) anterior border, beneath which a notch occurs. *For interesting descriptions and figures of Purbeck and Wealden forms see T. Rupert Jones, Quart. Journ. Geol. Soc., 1885, p. 311. Carboniferous to Recent. Common in Carboniferous. Cythere (fig. 124, c).-Shell thick, oval, or somewhat rectangular; often very highly ornamented with knots or spines. Right valve with a tooth at each end of the hinge-line, and a pit and horizontal groove between them; left valve with terminal pits, a ridge, and an anterior tooth. Gotlandian to Recent. Primitia. Shell small, thick, elongated oval; but hinge-border straight. Not always equivalve. A furrow runs on the surface from the hinge-border vertically towards the ventral border, sometimes reaching as far as the centre of the valve. Cambrian to Carboniferous. Especially Ordovician and Gotlandian. Beyrichia (fig. 125).-Shell thick, surface distinctly convex; straight hinge. Somewhat truncated anterior and posterior borders, and convex ventral border. Surface divided by strong fairly vertical furrows into markedly convex lobes, which commonly unite below; the marginal area is smoother. Cambrian to Carboniferous. Especially Ordovician and Gotlandian. Leperditia (fig. 126).-Shell large (attaining 2 cm. in length), thick, rather bean-shaped; straight hinge-border, and convex ventral border. Anterior border shorter than posterior. Right valve larger than, and lapping somewhat over, the left. A small eye-spot occurs near the hinge-border. Cambrian to Carboniferous. Especially Ordovician and Gotlandian. B. PHYLLOPODA. The animal is more distinctly segmented than in the Ostracoda, and the thoracic limbs, often numerous, are flattened and leaflike, each dividing into two flaps at the end. Many genera have a covering that recalls the ostracod shell. Compare also the Phyllocarida. Estheria.-Shell bivalve, thin, sometimes partly calcified, but commonly horny, with a polished appearance. A small rounded umbo occurs near the anterior margin; hinge-line toothless and straight. Ventral border convex. Surface in most species concentrically ribbed; sometimes smooth. Prof. T. Rupert Jones* remarks that, while Estheriæ have * Fossil Estheria. Palæont. Society, 1862, pp. 12 and 13. often been mistaken for small forms of Avicula and Posidonomya, their horny appearance will distinguish them from the former, while there is also an absence of any bending out of the concentric markings towards an ear, such as Avicula possesses. The valves, moreover, are rarely so quadrate in form as those of Posidonomya. Fresh and Brackish water; but early forms are found associated with Marine fossils, perhaps owing to floods, perhaps through difference of habit. Devonian to Recent. Fairly common in the Trias. C. TRILOBITA. Owing to the absolute extinction of the whole order, the relations of the Trilobites to the well-defined Crustacea and to the Arachnida have remained somewhat obscure. Thanks to the work of Messrs. E. Billings,* C. D. Walcott,† and J. Mickleborough, the appendages of the genera, Asaphus, Calymene, Cheirurus, and Acidaspis, are now fairly known; while Mr. Beecher has examined them in Trinucleus and, with signal success, in Triarthrus.§ Dr. Oehlert has recently summarised the results of these researches (Bull. Soc. géol. de France, 3me sér., t. xxiv., p. 97); and it has been pointed out that Linnæus observed antennæ in a specimen of Parabolina as far back as 1759 (see Geol. Mag., 1896, p. 142). The verification of these in Triarthrus has led to the retention of the trilobites among the Crustacea; and Mr. H. M. Bernard sees in Apus their nearest living representative. The hard covering of the trilobites, which is all that ordinarily remains to us, can be clearly divided into three parts, the nomenclature of which has depended upon the classificatory position taken up by successive palæontologists. The supporters of the arachnid view employ "cephalo-thorax," "abdomen," and "postabdomen" or "pygidium"; the rival school uses "head," "thorax,” and "pygidium." To avoid confusion, and to carry forward the same nomenclature when writing of the Merostomata, we propose to speak of the "head-shield," "body," and "pygidium." This is practically the plan adopted by Mr. H. M. Bernard. Head-shield (fig. 127). This portion is approximately semicircular, and is not broken up in the adult into transverse Quart. Journ. Geol. Soc., vol. xxvi. (1870), p. 479. + Bulletin Museum Comp. Zoology, Cambridge, U.S.A., 1881. § Am. Journ. Sci., 1895 and 1896. segments. It has curved anterior (or frontal) and lateral borders, forming an outwardly convex margin; while it has an almost Fig. 127.-Trilobite (Dalmania caudata; Wenlock Beds). H, Head-shield; T, Body-segments; P, Pygidium. e, Eye. fx.c, Fixed cheek. fr.c, Free cheek. fs, Facial suture (starting in this example from the lateral margin and finally passing round in front of the glabella). g, Glabella (bearing lateral furrows). pl, Pleura. straight posterior or occipital border, where it joins the first body-segment. An occipital furrow often occurs in the headshield parallel to and near this border. The shield is folded over below its front margin, so as to extend a little way back towards the mouth. In young forms, the original five-or, in some cases, four-segments composing the head-shield have been traced. Glabella.-The convex elevated portion of the head-shield reaching from the centre of the occipital border nearly to the anterior border. The glabella varies in form, and sometimes bears lateral furrows, which run approximately at right angles to its sides; they thus divide its edges into lobes, and probably represent traces of original segmentation. Facial Sutures.- Fine lines of junction between the two parts into which the area on either side of the glabella can be divided. Each of the two facial sutures arises at some posterior point of the border of the head-shield, runs forward between the glabella and the eye, and either terminates by cutting across the anterior border (fig. 128), or by meeting its fellow-suture in front of the glabella (fig. 132). Occasionally in the former case an additional suture, the marginal suture, runs from one facial suture to the other along the anterior part of the head-shield. This occurs in Calymene, Paradoxides, and Illænus. Fixed Cheeks.-The areas on each side of the glabella between it and the facial suture. Free Cheeks. The portions of the head-shield between the facial sutures and the outer margin. These two parts may become detached after the death of the animal, and are often seen to have shifted away slightly from the fixed cheeks. Eyes. These are sometimes absent (fig. 130), sometimes represented only by papille; but they are commonly present as two somewhat crescentic elevations, occasionally supported on a stalk, and are covered with numerous facets, each of which was a lens (figs. 127 and 135). As above hinted, they occur on the free cheeks, close against the facial suture, where it approaches the side of the glabella. Hypostome. The anterior portion of the head-shield is bent over, and may form a broad crescentic plate-like surface on the under side, between the anterior end of the animal and the position occupied by its mouth. A small plate, of various form, often occurs before the mouth, with its anterior border in contact with the edge of the folded-over head-shield, and its other borders free. This is the Hypostome (fig. 133), representing the labrum that overlaps the front of the mouth in higher crustacea. Body. The portion between the head-shield and the pygidium. It consists of a very variable number of segments, which were movable on one another, so that in some genera the animal could coil itself up after the manner of a wood-louse. Two longitudinal furrows or depressions run down the body, one starting on each side of the glabella, and corresponding, in fact, to the depressions which divide the glabella from the cheeks. Each body-segment is thus marked out into a central convex part, the annulus, and a flatter and commonly broader portion on each side. The latter areas form the pleura;* they are generally marked with a groove, or a ridge, from the annulus outwards, and often terminate in spines (fig. 128). The convex ridge formed by the series of annuli, running from the glabella over the body, and commonly on to the pygidium itself, is termed the rachis. Pygidium. The shield covering the posterior part of the trilobite. Its outline often repeats that of the head-shield (figs. 129 and 133), and it consists of permanently united segments. Sometimes the traces of the original segmentation are perfectly clear (fig. 134), and the rachis generally persists on it for some distance. * Pleura and pleura have alike been used; their respective singulars are pleuron and pleura, both of which are good Greek forms. |