[458] See Quart. Journ. Geol. Soc. vol. viii. p. 386.
Several fossil genera of the Entomostracous Crustaceans belong to the Phyllopoda (leaf-feet), which constitute a subdivision of the Branchiopoda (gill-feet). Of these perhaps the most remarkable is the Dithyrocaris, first discovered by Dr. Scouler in the Carboniferous shale near Paisley. This genus is allied to the recent Apus, and, like it, has a broad, flat, thin carapace, easily divisible down the middle of the back, and a lengthened tail or post-abdomen, with a trifid termination. Six species have been found in the Carboniferous deposits of the British Isles.
Ceratiocaris and Hymenocaris, which are related to the recent Nebalia, are also of the Phyllopod group, and maybe said to resemble a shrimp-like animal partly enclosed in a bivalve carapace, while its tail is exposed, and either protruded, or turned beneath the body. Of the first-mentioned genus there are three Upper Silurian species; and of Hymenocaris Mr. Salter has described a single species (H. vermicanda) from the Lower Silurian (Cambrian) of North Wales.
The Limnadiadæ,—another Phyllopodous family, of which the recent Limnadia is the type,—are well represented in the older rocks. These Crustaceans are also bivalved, but the body is wholly enclosed. They are of small size. Estheria, a genus closely related to Limnadia, occurs plentifully in the Wealden of Sussex and Germany, and in the Lias of Westbury. Mr. Bean also has noticed a species (E. concentrica) in the Oolite shell of Gristhorpe Bay; and the E. minuta[459] is characteristic of the Keuper division of the Trias in England and Europe.
[459] This was formerly described as a Posidonomya.
Leperditia and Beyrichia are other fossil genera belonging to the Limnadiadæ. These little crustaceans were gregarious in their habit, and, like Estheria, occur locally in great profusion. They are characteristic of the Silurian deposits; the former being an abundant Upper Silurian fossil in Europe and North America, and the latter,—a very minute form,—both in the Upper and Lower Silurian.
The next group of Entomostraca that we have to notice belongs to the Lophyropoda (feet crested with bristles), namely, the Ostracoda (shelled). The recent genera, Cypris, Cythere, and Cypridina,[460] are types of the three families of this group, and are the existing representatives of numerous closely related forms that occur in a fossil state.
[460] The student should consult Dr. Baird’s elegant and elaborate work on the Natural History of the British Entomostraca (published by the Ray Society), for information on the characters and habits of these interesting little crustaceans and their numerous allies