86 Mr. T. R. Jones on some species of Leperditia. 



Valves most convex at the middle and somewhat posteriorly ; 

 the posterior half of the valve broadest and boldly rounded 

 backwards and downwards. Old individuals less tapering ante- 

 riorly, and more oblong, than younger valves. Anterior and 

 posterior angles of the dorsal edge prominent in well-preserved 

 specimens, the extremities of the hinge-margin being truncate, 

 and the anterior and posterior margins of the valves obliquely 

 rounded off from beneath them. 



Ventral margin of the right valve overlapping that of the left 

 to the extent of about ^th the breadth of the carapace. 



Dorsal margin of the left valve slightly thicker than that of 

 the right, which latter is somewhat thickened only at its anterior 

 third; ventral margin inverted suddenly, and marked with 

 transverse strise within and without. 



Surface of each valve slightly depressed along a narrow area 

 just within the margin, which forms a gently raised convex 

 border, narrow on the dorsal edge, broader on the ventral. 



Central spot and radiating canals well defined ; the latter are 

 seen, in the cast represented at fig. 4 «, to spring from a circular 

 vascular impression, or sinus, surrounding the central spot or 

 tubercle. The radiating canals have left only obscure traces on 

 the cast* of the small specimen, fig. 5 a; which, however, shows 

 the reticulated or warty appearance of the central tubercle very 

 clearly. Anterior tubercle prominent, on an ill-defined slight 

 elevation, which is bounded behind by a faint sulcus extending 

 from the dorsal edge to the central tubercle. 



The smaller cast, fig. 5 a, which I regard as representing a 

 younger individual of this species, presents a broadish depressed 

 margin on the anterior and ventral borders, — a feature that 

 becomes almost obsolete in the adult specimens. It also bears 

 a gently curved crenulated ridge, of slight elevation, crossing 

 obliquely the postero-dorsal region of the valve ; — this ridge is 

 not traceable in the older specimen, fig. 4 a. 



Surface smooth, shining; colour usually dark brown; two 

 large, somewhat worn, odd valves in Mr. Morris's collection are 

 light brown in colour ; and in the largest the anterior tubercle is 

 whitish, with a central black spot (fig. 1 a, c). In these two old 

 individuals the surface exhibits traces of a fine punctation. 



This species occurs in the Upper Silurian limestone of Goth- 

 land f, where it appears to be not uncommon. Hisinger gives 

 the localities thus : — " In the Transition limestone of Gothland 

 at Lanna, near Slitehamn ; also in detritus on the sea-shore : 

 in brownish limestone in the Island Bjorko, near Holmestrand, 



* The surface of the cast is covered by very fine but obscure pittings, as 

 if the inside of the valve had been marked by an ill-defined reticulation. 



t For a notice of the geology and fossils of Gothland, see Quart, Journ. 

 Geol. Soc. vol. iii. pp. 18, 30, ike. 



