NATURAL HISTORY SOCIETY OF GLASGOW. 171 



margin. The distinctive feature of our specimen, namely, the 

 cervical spine, is not found in any member of that family. Similar 

 sj)ines are present in Trimideus concentricus, Eaton ; and Triarthrus 

 spinoms, Billings — both American species; the latter, also an 

 American genus, possessed, moreover, the additional peculiarity 

 of a spine projecting from one of the dorsal axial body rings. In 

 CyjjJmsjns only the latter spine is present. 



In the absence of the eyes, body, and tail — or rather, while it is 

 impossible to identify any of the still undetermined bodies with 

 this head, it is safest to leave it as a species of Cheirurus. It may 

 hereafter be found to form the type of a separate genus, but the 

 analogy of the TrinucUiis above-mentioned, forbids the separation, 

 on account of the cervical spine. 



I propose to name it Cheirurus trisjnnosus. Locality — Silurian, 

 Penkill, AjTshire. 



The small specimen, Plate I., Figs. 7a, 7b, presents, at first sight, 

 all the characters of a bivalve shell of the Solenoid group; but 

 Mr Young detected at the posterior part of the hinge line the 

 peculiar ornament which is comparable only to that found in 

 Crustaceans, and is never found in Lamellibranchs. 



The test is o1)long quadrilateral, one cjuarter 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, of which the lower is slightly the smaller, by a shallow 

 furrow 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 traversed 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 

 ujiper 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 surface, bounded 

 superiorly by the slightly thickened dorsal margin. This area is 

 traversed by famt concentric strise, 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 



