10 FOSSIL MALACOSTRACOUS CRUSTACEA. 



Family— LEUCOSIAD^E. 



Genus — Hemioon, Bell. 



Char. Gen. Testa ovalis, elevata, lateribus declivibus, antice producta, truncata. 

 Orbitce parvae, cylindricse, antrorsum aperta3. Oris apertura anguste ovata. 



Species unica. Hemioon Cunningtonii, mi/ii. Plate II, figs. 4 — 7. 



Descr. The carapace is of an elongate oval form, somewhat produced anteriorly, the 

 front truncated, the sides sloping, very convex from side to side, nearly straight from 

 before backwards ; the surface very smooth, extremely minutely granulated, the regions 

 very slightly indicated ; the lateral margin forming a continuous and even curve. The 

 orbits are placed on the outer side of the fronti-orbital opening, and are nearly cylindrical ; 

 and the eyes must have had a forward direction, as in the rest of the family. The oral 

 aperture is very narrow, ovate, elongate, narrowing forwards, and open to the antennary 

 fossa, without any intervening epistome. The Pterygostomian process obliquely sulcated. 

 From the size of the basal joints of the legs which remain in a small specimen in my 

 possession, it appears that the limbs must be somewhat robust. The abdomen in the 

 male is narrow, linear, and the only segments which remain, the first four, are all separate. 



Length of the carapace, 0*6 inch; breadth, 04 inch. 



Two specimens exist in Mr. Cunnington's collections from the upper Greensand of 

 Wiltshire, a very small one in that of Mr. Carter, of Cambridge, and one in my own ; 

 both these are from the upper Greensand of Cambridge. 



05s. The peculiar interest which attaches to the present species consists in its being 

 the only example of a fossil Leucosian hitherto discovered in this country, if we except 

 Ebalia Bryeri, a not uncommon living species inhabiting our coasts, of which Mr. Searles 

 Wood found some remains in the Coralline Crag. Two species, belonging undoubtedly 

 to the family Leucosiadae, are described by Desmarest.* Of the first of these, Leucosia 

 cranium, the author says, " Le mode de conservation de cette fossile est le meme que celui 

 que presentent les especes qui viennent des Indes orientales." This is the only indication 

 given of its locality. Of the second species, L. subrhomboidalis, no locality is mentioned. 

 There is a third species, described by the same author under the name Leucosia Prevostiana, 

 which does not appear to belong to this family. 



The form of the carapace is very peculiar, and differs from that of the Leucosiadse 

 in general in its proportions, being almost twice as long as it is broad, a proportion 



* 'Hist. Nat. des Crust. Foss.,' p. 112 et seq., pi. ix. figs. 10—13. 



