There are two species that appear to have some relations with Corystes, a genus which includes several recent crustaceans that inhabit our shores, and are characterised by their elongated oval shell and four antennæ, the external pair being long, setaceous, and furnished with two rows of cilia. The tail is folded under the body when the animal is in repose. They have ten legs, the anterior pair chelate (with pincers), the others terminating in an acute elongated nail or claw. The fossils consist of the carapace, and one example possesses the inferior or thoracic plates and the remains of the bases of some of the legs (see [Lign. 168], fig. 3a).
Notopocorystes Stokesii. [Lign. 168, fig. 2.]—The carapace is relatively wider than in most species of this genus; is has a strong dorsal ridge of irregular oblong tubercles; the union of the cephalic and thoracic segments is marked by a transverse undulated groove; there are three or four tubercles on the surface of each lateral portion of the former, and one on each of the latter. The whole surface is finely granulated. The openings left by the attachment of the peduncles of the eye remain.
Notopocorystes Broderipii. [Lign. 168, fig. 3.]—This species, like the former, has a transverse undulated furrow, indicating the union of the cephalic with the thoracic segments; the dorsal ridge is smooth, and there are two tubercles on each lateral cephalic portion of the shield. The carapace is longitudinally ovate, much depressed, with three sharp points directed forwards on each margin of the anterior part: the whole surface is finely granulated. In the specimen fig. 3a the sternal plates, with portions of the first joints of the claws, remain; one example (figured Geol. S. E. p. 169) possessed six or seven arcuate abdominal segments, which were turned under the body.[441]
[441] I have described these small crustaceans somewhat minutely, and have given them specific names, in the hope of directing the attention of collectors to these highly interesting relics, and leading to the discovery of more illustrative examples. See Foss. South D. pp. 96, 97.
The carapace or shell of the other crustacean observed in the Sussex Galt (Notopocorystes Bechei) is of an orbicular inflated form (see Geol. S. E. p. 169, fig. 3), and ornamented with twelve or thirteen aculeated tubercles; its margin is dentated.
In the friable arenaceous limestone of the Cretaceous formation at St. Peter’s Mountain, near Maestricht, the cheliferous claws of a small kind of crustacean (Mesostylus Faujasii, Wond. p. 338), are frequently discovered (and occasionally in the Chalk of Kent and Sussex), but with no vestige of the carapace or shell. This curious fact is explained by the analogy existing between the fossil claws and those of the Pagurus, or Hermit-crab, whose body is only covered by a delicate membrane, the claws alone having a calcareous covering; hence the latter might be preserved in a fossil state, while no traces of the soft parts remained. In the fossil, as in the recent claws, the right arm is the strongest. There is no doubt that the crustaceans to which the fossil claws belonged possessed the same modification of structure as the recent (anomurous) Hermit-crab, and must have sought shelter in the shells of the mollusks with which their durable remains are associated.
FOSSIL LOBSTERS.
Fossil Lobsters.—The macrurous, or long-tailed, crustaceans, as the Lobster, are distinguished from those of the former divisions by the prolonged abdomen (or tail, as it is commonly termed), which forms a powerful instrument of locomotion, and enables the animal to dart backwards through the water with great rapidity; and this is furnished with an appendage or tail, which none of the ambulatory crustaceans possess.
Of the fresh-water species, the Cray-fish (Astacus fluviatilis), and of the marine, the Lobster (Astacus marinus), are illustrative examples. The remains of three macrurous species occur in the London Clay of the Isle of Sheppey, associated with congenerous crustaceans; and the segments of the tails (post-abdomen) are often well preserved.