172 PROCEEDINGS OF THE 



vertical margin is everted; the remainder of the antero-ir^^rior 

 area is in one plane. The middle of the postero-superior area 

 projects considerably, in consequence of the prominence of the 

 elevated ridge, the upper surface being flattened slightly towards 

 the hinge line; but posteriorly the surface is flattened very 

 markedly towards the posterior rounded margin, so that the two 

 valves would be approximated pretty closely. 



Its resemblance to a gaping bivalve is thus considerable, but on 

 closer comparison the diff"erence will appear well marked. No 

 Lamellibranch has the anterior greater than the posterior gape; 

 when the relation is not one of equality, the posterior is the larger. 

 Orthonotus, the nearest in form, is not one of the gapers, nor is its 

 hinge line linear with parallel bounding areas; its oblique, lateral 

 ridges, moreover, cut the inferior margin, not the posterior outline, 

 as in the specimen before us. 



The ornament, again, is not that of Lamellibranchs; its granular 

 surface has a corresponding obverse surface seen when the shell 

 has iDeen removed, and exactly similar to that in the internal casts 

 of Dithijrocaris. The lateral position of the beak removes the 

 Brachiopoda at once from consideration, though, in some respects, 

 the outer and inner surfaces present appearances not unlike those 

 of Procluctus. 



No other conclusion is open except that this form is a crustacean, 

 and, so far as I am aware, of an undescribed type. Those which 

 are known from the Silurian rocks are Hymenocaris, Peltocaris, 

 Discinocaris, and Ceratiocaris. In all these the extended carapace 

 forms a shield, with rounded outlines. Our specimen would give 

 a quadrilateral shield. The character of the ornament is wholly 

 peculiar; no less so is the want of the definite emargination seen 

 in the Crustacea above-mentioned. 



The absence of distortion in any of the other fossils from the 

 same locality, and the presence of the eversion of the anterior 

 margin in all the specimens, forbids the supposition that this 

 character is accidental. 



The beds from which the specimens were obtained are in the 

 Silurian series in Penwhapple Glen, near Girvan. The determina- 

 tion of their position is still under consideration by the officers of 

 the Geological Survey — the difficulties in the way of a decision being 

 partly stratigraphical, partly due to tlie remarkable mixture of 

 fossils, which in England belong to distinct stages. The burden of 



