II. Family Portuntdae. (Gen. nov. Rhachiosoma.)
Amongst the pelagic Crustaceans we find numerous examples belonging to the Portunidae, all armed with long spines on the hepatic region, and with the lateral borders of the carapace greatly produced. Thus the genera Matuta, Orithyia, Podophthalmus, Portunus, Lupea, and many other forms possess long hepatic spines.
Two Eocene genera have also been described and figured by Dr. Alphonse Milne-Edwards in his 'Histoire des Crustaces Podophthalmaires Fossiles,' namely, Enoplonotus armatus, from the Nummulitic beds, Salcedo, and the Psammocarcinus Hericartii (Plate IV. fig. 4), from the Sables de Beauchamp (Lower Eocene). We are now, by Mr. Meyer's exertions, made acquainted with two new forms (see Plate IV. figs. 3 and 5) from the Lower Eocene of Portsmouth, which it is proposed to place in a new genus, the characters presented by the carapace in the specimens under consideration not warranting us in referring them with certainty to any genus of fossil Crustacea already established.
Rhachiosoma*, gen. nov.
Carapace produced laterally into two more or less long and pointed spines ; latero- anterior border also furnished with spines ; surface of carapace tuberculated.
1. Rachiosoma bispinosa, sp. nov. (Plate IV. fig. 3.)
This form is remarkable for the great development of its two lateral spines, which in length exceed half the breadth of the carapace. In section they are nearly round, slightly recurved at their extremities, and taper gradually to a point. The carapace itself measures 1 inch in length and 1-1/4 inch in breadth (exclusive of the hepatic spines, which are each 10 lines in length).
The cardiac region is separated from the branchial regions by two undulating subcentral furrows, and bears a single tubercle upon its centre. The gastric region is ornamented with two small subcentral tubercles. Two prominent equidistant tubercles mark the centreline of the branchial region, and form, with a third on the mesogastric region, a prominent ridge on either side the mesial fine of the carapace, inclined towards the rostrum at an angle of about 80°. A solitary tubercle on the hepatic region, just in front of the base of the great hepatic spine, completes the ornamentation of the surface of the carapace.
The latero-anterior border appears to have been armed with two or more marginal spines; but the intense hardness of the matrix (a fine-grained quartzite) in which the specimen is imbedded has rendered its development unsuccessful. The posterior border of the carapace is half an inch broad.
The surface of the carapace (where preserved) shows it to have been very minutely and delicately punctate.
- From _____ and _____.
