A few species of Limulus, several Cytheres and allied genera, and a few Trilobites (Phillipsia and Griffithides) belong to the strata of the Carboniferous System. In the Devonian System we find some minute entomostracans, the gigantic Pterygotus, and various Trilobites (Brontes, Cheirurus, Homalonotus, Phacops, &c.); but it is the Silurian rocks that constitute the grand mausoleum of those ancient beings, the Trilobites.
There are about thirty genera of Trilobites found in the Silurian rocks of Great Britain and Ireland. Many of these are common to the Upper and Lower Divisions of that system; and some of them are met with both in the Silurian and in the Devonian rocks, as Phacops, Brontes, Cheirurus, Harpes, and Homalonotus. The Calymene Blumenbachii ([Lign. 175], figs. 3 and 4) ranges through the Ludlow and Wenlock, to the Bala and Llandeilo formations. The Phacops caudatus also ([Lign. 177]), the Cheirurus bimucronatus,[492] and the Encrinurus punctatus[493] extend from the Ludlow, to the Llandeilo formation. The Upper Silurian rocks exclusively contain some peculiar forms, as Encrinurus variolaris, Bumastus Barriensis, and several species of Acidaspis. And the Lower Silurian has several distinct genera, namely, the Trinucleus ([Lign. 175], fig. 2), Ogygia, Agnostus, Asaphus, Olenus, Remopleurides, &c. One species of Pterygotus, and one of Eurypterus, the Ceratiocaris and Hymenocaris, and several species of the minute bivalved Entomostraca (Leperditia and Beyrichia) are all that remain to be enumerated as constituting, in company with the Trilobites, the Crustacean fauna of the ancient Cambrian and Silurian seas.[494]
[492] In Murchison’s Silurian System this form is figured (pl. xiv. figs. 8 and 9) and described as Paradoxides bimucronatus.
[493] This is described and figured as Asaphus tuberculatus in Buckland’s Bridgewater Treatise, pl. xlvi. fig. 6.
[494] We have also to refer to the indications of the existence of other large Silurian Entomostraca afforded by the magnificent series of fossil foot-tracks lately brought to England by W. E. Logan, Esq., and obtained by that gentleman from the Potsdam Sandstone (Lower Silurian) of Eastern Canada. These foot-marks and trails have been determined by Prof. Owen as being most probably referable to some large Crustaceans of the Limulus Group, and are named by him Protichnites. (See drawings and descriptions in the Quarterly Journal of the Geological Society, vol. viii.)
On Collecting Fossil Crustaceans.—The Crabs and Lobsters of the argillaceous tertiary strata are generally imbedded in nodules of indurated clay and septaria. On the shore beneath the cliffs on the north of the Isle of Sheppey, and near Southend, specimens may be observed in the nodules that have been exposed to the action of the waves, the attrition to which they have been subjected having partially worn away the surrounding stone, and displayed the enclosed fossils. In these examples the carapace is occasionally seen on one side, and the pair of pincer-claws on the other face of the boulder; the other feet and the plates of the thorax may sometimes be developed in such examples by chiselling away the enveloping mass. In the laminated marls of the tertiary and other deposits, in which the minute crustaceans, as the Cyprides abound, thin slabs covered with these relics may be easily extracted; and many of the tertiary clays and sands yield Cytheres, together with Foraminifera and other minute fossils, on careful washing and examination with a lens.
The Chalk crustaceans, particularly those which are muricated, or beset with spines and tubercles, as the Enoploclytia Sussexiensis and E. Leachii ([Lign. 169]), require considerable patience and dexterity to develope successfully. The crustaceous covering of the carapace and claws adheres firmly to the chalk by the rough external coat, while the inner, smooth, glossy surface as readily separates. Hence, upon breaking a block of chalk containing portions of these crustaceans, we find one piece exhibiting a chalk cast of the claw or carapace, covered with tubercles or papillæ, that have been moulded in the bases of the spines of the crust; and on the other portion the crustaceous shell imbedded by its outer surface, and presenting the internal glossy lining, beset with circular depressions, which are the bases of the spines. This crust is exceedingly friable, and will flake off by a very slight touch. To obtain specimens with the external characters, it is necessary to proceed with great caution; and when indications of a crustacean are observed in a block, the chalk should be chiselled or sawn off to within half an inch of the surface of the fossil, and the remainder of the stone be cleared away, piece by piece, by means of a penknife or graver. By this process the fossils figured Foss. South D. pl. xxix. xxx. xxxi. were developed. When a fine specimen has been broken, and the shell is attached to one piece of the stone and the cast to the other, it is possible to obtain an illustrative example of the external surface, by cementing the pieces accurately together with very thin hot glue; and, when firmly consolidated, the chalk may be removed, and the spines, tubercles, and papillæ of the crustaceous covering be developed by the method previously described. A thin coating of mastic varnish will give durability to the crust, and improve its appearance; but the rich brown colour it possesses when first exposed soon disappears. The Crustaceans of the Galt are often found amongst the argillaceous and pyritous nodules flung aside in heaps where the Galt is used for brick-making.
The Cytheridæ of the Chalk, Galt, Oolite, &c. are to be obtained by disintegrating the matrix in water, and examining the debris, after sifting, under a lens.
The Limuli of the Coal-measures often form the nuclei of clay nodules, as in the example figured [Lign. 172], in which fig. 2 represents the nodule without any external indication of its contents, and figs. 1, and 3, the same broken, and displaying the crustacean. Traces of the legs, branchiæ, and other appendages, should be diligently sought for in fossils of this kind, for they are more likely to be detected in such specimens than in those found in limestone. It is possible that polished sections of the coiled up examples of Trilobites ([Lign. 175], fig. 4) would throw some light upon the nature of the hitherto undiscovered organs of locomotion and respiration of this extinct order of Crustaceans.