PLAGIOPHTHALMUS. 9 



Family— PINNOTHERIDiE. 



Genus — Plagiophthalmus, Bell. 



Char. Gen. Testa oviformis, valde convexa, laevis, margine omnino integerrimo. 

 Orbita minimse, elongatse, obliquae, intra testae rnarginem positse. 



Species unica. Plagiophthalmus oviformis, mild. Plate II, figs. 1 — 3. 



Descr. Carapace evenly egg-shaped, very convex, the height from the plane of the 

 lateral margin being equal to nearly half of the length ; the front slightly produced and 

 bent downwards ; the surface smooth ; the nuchal furrow shallow, and a second furrow, 

 nearly parallel with the former, extending across the carapace between the meso- and 

 meta-branchial lobes and across the cardiac region ; a small, bifurcate, impressed line 

 passing backwards from the front for a short distance on the carapace. Orbits very 

 small, elongate oval, placed obliquely within the margin, and appearing as if pierced in 

 the substance of the carapace. In the two specimens observed there is nothing remaining 

 but the carapace, which is tolerably complete in each. 



Length of carapace, 06 inch; breadth, - 5 inch. 



There are two specimens from the upper Greensand of Wiltshire in the collection 

 of Mr. Cunnington, and three in the British Museum from the same locality. 



This is certainly one of the most interesting fossil species I have yet met with. Its 

 form is very remarkable, being as nearly as possible that of half an egg, a figure which at 

 once recalls that of Retnipes, a resemblance, however, which is not borne out by any im- 

 portant characters. It is very unlike the figure of any species with which its essential 

 characters would appear to associate it ; but the uniformity and smoothness of the 

 carapace, the slight indications which exist of regional distinctions, and, above all, the 

 form, situation, and direction of the orbits, appear to me to indicate a near approach to 

 certain forms of the Pinnotheridae. The genera to which I consider it as most nearly 

 allied are Xenophthahnus of White and my genus Amorphopus. In the last-named form 

 the orbits are placed within the frontal margin of the carapace, but open forwards where 

 the sides slightly approach each other anteriorly ; in Xenophtlialmus the orbits are ex- 

 tremely small, placed far within the margin, and the sides of the orbit completely close in 

 front, so as to have given to the original describer of the genus, the idea that the orbits 

 were actually pierced through the carapace. In the present equally remarkable form the 

 orbits appear prima facie to be absolutely excavated in the substance of the carapace. 



