Appendages. Of the five pairs of head-appendages known, the first is a pair of antennæ, their bases inserted on the edge of the hypostome. These are long in Triarthrus, and consist of one ray. The bases of the remaining four pairs were probably used in mastication. Each body-segment bore a pair of appendages, and the pygidium had also a number of pairs corresponding to its original segmentation. All these limbs were fixed to the inside edges of the rachis. They were simple biramous types; and the pygidial pairs, and possibly those on the body, served for respiration as well as for swimming. There is a gradual change in type, from the anterior body-appendages, which resemble those on the head-shield, to the posterior ones, which resemble those on the pygidium.

Lastly, we should note that the more resisting character of the head-shield and the pygidium often allows of their being found isolated in rocks, when the body-segments have become parted asunder and lost. It has been suggested that the "trilobite," as found, may often be a mere "skin cast off by the animal during life.

The Trilobites were all Marine.

Paradoxides. Form elongated; sometimes large (70 cm. or so in length), tapering fairly uniformly from the front to the pygidium. Head-shield semicircular, with a long curving spine running backwards from each of its posterior angles. Glabella rather flat, rounded and broad in front, narrowing posteriorly, with lateral furrows. Facial sutures running from posterior to anterior border, without bending in any great degree towards the glabella. A marginal suture is present. Body with numerous (16 to 20) segments, and with well-marked trilobed character. Pleura furrowed and prolonged as spines. Pygidium very small, the rachis being continued on to it for a short distance; a long spine often runs out posteriorly on each side.

Exclusively Cambrian.

Typically Middle Cambrian. Olenellus. Resembles Paradoxides, with narrower glabella; facial sutures obscure or absent. Third body-segment often larger than the others. Pygidium at times styliform. 13 to 26 body-segments.

Lower Cambrian.*

* See "The Story of Olenellus," Natural Science, vol. i., p. 340. Holmia and Mesonacis are subgenera.

Olenus (fig. 128).-Form broader and more oval than in Paradoxides. Head-shield broad and semicircular, with a spine running back from each posterior angle. Glabella rather conical, narrowing anteriorly, and with lateral furrows. A little ridge runs out from it to each eye, at right angles to the axis. Facial suture running from posterior to anterior border, sometimes closely approaching glabella. Body with 12 to 15 segments, which have short sharp backward terminations. Pleura broad. Pygidium small, broad, with rachis well marked on it for some distance.

Exclusively Cambrian. Typically Upper Cambrian.

Conocoryphe (Conocephalus).-Form much like Olenus. Headshield semicircular, without posterior spines. Glabella conical, furrowed; somewhat truncated at its narrower (anterior) end, and divided by deep depressions from the cheeks. Eyes rather near the anterior end of the glabella, and facial sutures running from the posterior margin, near the angles of the shield, in wards almost to the glabella, and then out, widely diverging, across the anterior border. Body with fourteen or fifteen segments; rachis well marked; pleura furrowed, and rounded at the ends. Pygidium small, with distinct rachis, and with traces of segmentation. L. Cambrian to Ordovician.

Angelina.-Proportions much as in Olenus, and head-shield with posterior spines. Glabella narrowed anteriorly, but smooth and rounded. Body with fourteen or fifteen segments; pleura furrowed.

Cambrian.

Agnostus (fig. 129).-Form small, elliptical, the head-shield

Fig. 129.-Agnostus Fig. 130.-Trinucleus concen

(Cambrian).

tricus (Bala Beds). In
various stages of growth.

and pygidium being almost similar; both are rounded at the outer end, with convex or straightish sides.

Glabella distinct.

No eyes or facial sutures. Body with only two segments. Pygidium with fairly marked rachis, which terminates broadly; a little process often runs out posteriorly from each lateral border of the pygidium; but it is otherwise difficult to distinguish detached pygidia from head-shields.

Cambrian.

Trinucleus (fig. 130).-Head-shield large and predominant, projecting laterally beyond the body; semicircular in front, and often with long spines from the posterior angles. Glabella and cheeks smooth, and forming three strongly convex elevations, which leave a broad flat semicircular border beyond them. This border is pierced with minute holes. No eyes or facial sutures. Body with six segments; rachis rather narrow and distinct, continued on to the small pygidium; pleura furrowed. The body and pygidium are together smaller in area than the head-shield. Ordovician.

Harpes (fig. 131). This remarkable form bears some resemblance to Trinucleus, having a similar broad perforated border to the head-shield, prolonged backwards in this case almost as far as the pygidium. Eyes present, but no (or very indistinct) facial structures. A little ridge sometimes runs from the glabella to each eye, as in Olenus. Numerous body segments (about twenty-five). Pygidium very small. The flat border of the head-shield is sometimes found detached and isolated. Ordovician to Devonian.

Calymene. Form oval, rather broad. Often ornamented with little tubercles. Head-shield broad, rounded anteriorly; posterior angles generally without spines. Glabella convex, with three strong pairs of furrows, the most posterior pair in some species bifurcating at the end. Facial sutures running from the posterior angles obliquely inwards to the eyes, and then across the anterior border, where they are connected by a marginal suture. Body with thirteen segments; rachis well marked. Pygidium rounded and scarcely distinct from the body, the rachis reaching to the end, and traces of segments being clearly marked.

This genus is one of those most frequently found in a rolled up condition, as in the specimens from the Wenlock Beds at Dudley. The hindmost of the pairs of limbs on the head-shield is larger and broader than the others, suggesting its differentiation into a special pair of paddles.

Ordovician and Gotlandian.

Homalonotus (fig. 132).-Form fairly long; sometimes ornamented with spines. Head-shield rather broad, either rounded or pointed anteriorly; no posterior spines. Glabella commonly only feebly marked off from the cheeks, and unfurrowed. Facial sutures much as in Calymene, but commonly meeting without intersecting the anterior border. Body with thirteen

Fig. 132.-Homalonotus (Gotlandian). Fig. 133.-Asaphus Powisii (OrdoShowing facial suture continuous

vician). With hypostome.

segments; rachis not sharply marked off. Pygidium with rachis and traces of segments; pointed posteriorly, sometimes ending in a short spine.

Ordovician to Devonian.

Ogygia. Form often large, roundly oval, and rather flat. Headshield semicircular, sometimes with posterior spines. Glabella rather straight at sides, widening in front, with four pairs of furrows. Hypostome not notched on its posterior border. Facial sutures running from the posterior border, near the angles, obliquely to the large crescentic eyes; thence they sometimes cross the anterior border, but generally unite in front of the glabella. Body with 8 segments; rachis well marked; pleura broad, furrowed, not spinose at the ends. Pygidium about the same size as the head-shield, and nearly semicircular, slightly

elongated; its rachis is distinct, with numerous segmental markings on it and on the lateral areas.

Ordovician.

Asaphus (fig. 133).-A close ally of Ogygia, and often large. Like Ogygia, but rather more convex; 8 body-segments. The traces of segmentation on the pygidium are confined to its rachis or altogether absent. The head-shield may be sharply pointed in front, or semicircular; glabella commonly not furrowed. Hypostome with a deep notch on posterior border. The glabella on the head-shield and the rachis on the pygidium may appear merely as broadly convex folds.

Ordovician.

Illænus. Ally of Asaphus, but commonly more strongly convex, and more broadly elliptical in outline. Glabella only feebly indicated, externally without furrows. Facial sutures connected by a marginal suture; 8 to 10 body-segments (typically the latter); pleura smooth. Pygidium with slight trace, if any, of rachis, and with no external signs of segmentation. Ordovician and Gotlandian.

Phacops (fig. 134).-Form elongated, oval, or elliptical. Headshield almost semicircular, without posterior spines. Glabella

much widened anteriorly, distinct; only feebly furrowed, except in the posterior part. Facial sutures arising on the lateral margins, almost opposite the eyes, and uniting in front of the glabella. Body with 11 segments. Pygidium semicircular, with marked rachis and signs of segmentation. See Dalmania.

Gotlandian to Devonian.