Trinucleus. [Lign. 175], fig. 2; Ly. fig. 432.—This genus comprises several small forms which are found in the Lower Silurian rocks of England, and occur in the equivalent deposits of Sweden, Norway, and Russia.[485] In the Trinucleus, the cephalic shield is obtuse, trilobed, rounded, and terminating in lateral spikes; and its margin is marked by numerous pit-like depressions. There are six body-rings or thoracic segments. The caudal shield is large and somewhat triangular. There are no distinct eyes.
[485] Murch. Sil. Syst. p. 217.
Paradoxides. [Lign. 178].—The Trilobites of this genus are easily recognised by the ends of the lateral segments of the thorax and abdomen terminating in deflected points, which extend in spikes beyond the membrane they supported, and particularly those near the tail, which are much elongated; whereas in the other genera the lateral points of the segments are united by a membrane, which often forms a border beyond them. The cephalic buckler is semicircular, and its lateral angles are lengthened out behind into two strong spines; it is divided on the median line into four protuberances, by transverse grooves. The thorax consists of from sixteen to twenty segments; the abdominal buckler is generally very small and rounded. The animals of this genus have the body much depressed, and the lateral lobes wider than the middle lobe: some species are of considerable size, attaining several inches in length.
Lign. 178. Paradoxides Bohemicus: nat.
Silurian. Bohemia.
A very peculiar form of Trilobite (Brontes flabellifer, Ly. p. 348) is found in the Devonian strata of the Eifel and South Devonshire; the head, or cephalic region, is narrow, and has two lunated eyes; the thoracic region is trilobed and short, and composed of about ten small articulations; the abdominal very small, and bordered by segments, which radiate and form a wide, fan-shaped expansion. Other species of this genus occur in the Silurian rocks.
With regard to the under surface of the Trilobites much remains to be known. No decided indications either of antennæ or extremities have been discovered. In an American specimen, Mr. Stokes detected a plate,[486] which appears to be a labrum, or upper mandible or lip, resembling that of Apus cancriformis. This animal has a similar labrum, "and lateral influted terminations of the shelly segments of the body, with a distinctly trilobed pygidium (tail or caudal portion), and a prolonged tail: the feet being foliaceous, and the abdomen merely covered by a membrane."[487] In the upper or dorsal surface of the carapace the Trilobites approach certain Isopoda, particularly in the characters of the buckler and eyes. Mr. Macleay states that among the existing crustaceans there are certain genera which individually possess some one or more of the characters, which have been thought peculiar to the extinct Trilobites. Thus the Serolis (Bd. pl. xlv. fig. 6), and the Bopyrus, have a trilobed form; the female Cymothoæ have the coriaceous margin of the body, and in some species are without eyes as are many of the Trilobites; while the eyes of the males of some Cymothoæ are composed of large facets, and are situated on the back of the head, wide apart, as in the Calymene; rudimentary feet, and the absence of antennæ occur in Bopyrus; and lastly, the Sphæroma has an onisciform body, and the power of rolling itself up into a ball, like the Calymene ([Lign. 175], fig. 4). The analogy between the Bopyrus and the Barr Trilobite is so close, that if the latter had a body with thirteen equal segments, and short crustaceous feet, it would be in every essential particular a male Bopyrus.[488] Burmeister regards the Trilobites as being related to the Branchipus. From the absence of eyes in the female, and their presence in the male of certain recent genera of crustaceans, it is not improbable that a similar character may have prevailed in the Trilobites, and that certain fossils referred to different genera, from the presence or absence of eyes, may have been the males and females of the same species.
[486] Geol. Trans, vol. ii. p. 208. See also Bd. pl. xlv, fig. 12; and Burmeister, pl. vi.
[487] Murch. Sil. Syst. p. 665.