﻿1851.] 



FORBES ON THE OOLITES IN SKYE. 



Ill 



Perna Murchisonii. Plate V. fig. la, 1 b, 1 c. 



Shell elongato-subtriangular, valves unequal but both tumid, an- 

 teally straight, very tumid and steep-sided, postealiy angulated, rapidly 

 declining to an acute margin, surface obsoletely wrinkled; dorsal portion 

 not expanded, ovate. Hinge-line very oblique, straight, terminating at 

 less than half the length of the shell, beaks acute ; young shells dor- 

 sally carinated ; pits of hinge distant, four or five in number. Length 

 1 inch T 3 y ths. Maximum breadth T 8 -2 ths of an inch. Thickness of shell 

 with valves united T \ths of an inch. This well-marked Perna differs 

 from any of the species of which I have seen examples from Brora, 

 and is equally distinct, or more so, from that which I have found in 

 the Purbecks. It is the "Mytilus ? " of Sir Roderick Murchison's list 

 (loc. cit.). I have dedicated it to that distinguished geologist, as the 

 discoverer of the Loch Staffin estuary strata. 



Trigonia tripartita. Plate V. fig. 11 a, Mb. 



Shell moderately tumid, ovato-subtriangular, obliquely carinated, 

 the carina nodulose ; the space between the carina and the posteal 

 margin is occupied by about three longitudinal radiating ribs, not 

 very elevated ; the central portion of the shell is ornamented with 

 eight or nine very oblique strong rounded ridges proceeding from the 

 nodulations of the keel towards the margin in an anteal direction, but 

 all stopping short except the two lowest, and forming acute angles 

 with a third and more numerous (about twelve) set of acute and 

 slender ribs which run very obliquely in the contrary direction, i. e. 

 from the anteal margin towards the centre of the shell. The teeth of 

 the hinge are strong and well marked. The largest valve found mea- 

 sured yyths of an inch from beak to frontal margin, by T 9 2 ths of an 

 inch maximum breadth. The height of a single valve was rather 

 more than yyths of an inch. I found only single valves of this 

 curious and very distinct Trigonia, an ally of T. undulata, and cha- 

 racteristically an oolitic form. 



Unio ? Staffinensis. Plate V. fig. 5 a, 5 b. 



I have given this name provisionally to impressions of a bivalve 

 having the form and aspect of a small Unio. It is transversely ob- 

 long, inequilateral, depressed, truncated anteally, rounded and nar- 

 rowed postealiy, and transversely sulcated. Its breadth is yyths of 

 an inch. Adult specimens will probably be found hereafter. 



Cyrena Jamesonii. Plate V. fig. 7 a, 7b, and variety, fig. Sa, 8 b. 



Shell moderately tumid, ovate, inequilateral, anteally declining, an- 

 teal extremity subcentral, posteal rounded but not abbreviated, surface 

 obsoletely furrowed by lines of growth, sometimes nearly smooth, 

 beaks subacute. Length of a fairly grown example (not the largest 

 found) 1^ ths, breadth 1 inch, and thickness y^ths of an inch. This is 

 an abundant shell. I cannot distinguish specifically between the two 

 forms, the one rather more tumid and narrowed postealiy, and the 



