from the Silurian of Girvan, Ayrshire. 357 



ornament wMcli is comparable only to that found in Crustaceans, and is never found 

 in Lamellibranchs. 



The test is oblong quadrilateral, one quarter of an inch in length, and one 

 fourteenth in breadth. The anterior extremity is truncated vertically, the posterior 

 is gently rounded. The outer surface, on which an eye-spot is visible,^ is divided 

 into two nearly equal parts, one of which, starting from the anterior superior angle, 

 cuts the lower margin in front of the lower posterior angle. The area anterior and 

 inferior to this furrow is ti-aversed by nine or ten longitudinal ridges, of which the 

 lowest is more elevated than the others, and forms a thickened rim to the test. The 

 posterior and upper triangular area is divided by a triangular ridge, which, narrow 

 at its anterior superior end, gradually widens backwards, and covers more than the 

 lower half of the posterior extremity of the test. Above this ridge is a gently convex 

 sm-face, bounded superiorly by the slightly thickened dorsal margin. This area is 

 traversed by faint concentric striae, the continuation of some of the coarser anterior 

 ridges. It is on this area that the ornament above mentioned is seen. It consists of 

 very close-set, minute tubercles, arranged in linear series opposite each other. 



The contours of the surface are in several planes. The anterior vertical margin is 

 everted ; the remainder of the antero-inferior 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 difference wUl 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. Urthonotus, 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 sheU has been removed, and exactly 

 similar to that in the internal casts of Bithyrocaris. 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 Productus. 



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 Ce^-atiocaris. 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. 



After a careful study of Messrs. Young's Solenocaris, and com- 

 parison with the Chiton-like series of plates above noticed, we are led 

 to conclude that there is a very general agreement between them, 



1 I have not been able to satisfy myself as to the presence of the eye-spot here 

 referred to, in any of Mrs. Gray's specimens. — H.W. 



2 We are reminded by the Solenocaris of Young of the small oblong bivalved 

 Crustacean carapaces, named by Prof. T. Eupert Jones Zeaia (see Appendix to a 

 Monograph of the Fossil Eslherim, by T. Rupert Jones, Pal. Soc. Mon. 1862, p. 116, 

 pi. V. figs. 11, 12, Leaia Zeidyi, Pennsylvania; L. Leidyi, var. WiUiainsoniana, 

 Jones, op. cit. p. 117, pi. i. figs. 19, 20, and var. Salteriana, fig. 21, from 

 Ardwick, Manchester, and Fifeshire ; others are known from Edinburgh, Bristol, 

 South Wales, Germany, Nova Scotia, and Illinois) ; but the sculpturing and orna- 

 mentation is, to all appearance, that of Estheria. 



See on some Bivalved Entomostraca from the Coal-measures of South Wales, by 

 Prof. T. Rupert Jones, Geol. Mag. 1870, Vol. VII. p. 214, PI. IX. with figures 

 of Leaia. 



See also on the Occurrence of a Phyllopod genus, Leaia, in the Lower Carboniferous 

 Rocks of Edinburgh, by R. Etheridge, juu. (Annals and Mag. Nat. Hist. 1879, 

 5th series, vol. iii. pp. 257-263). 



