ROBERTSON ON THE WEALDEN BEDS OF BRORA. 113 



posterior portion of eacii volution between the ends of the ribs and 

 the suture. Aperture oblong-ovate, both lips smooth, canal modeft 

 rately long, and curved to the right. Length '55 inch, breadth 

 •15 inch, jHrst volution *25 inch : angle of spire 30°. 



Mitra juniperus, Strickland. — Taper, with ten or 

 eleven slightly rounded volutions. On each volution 

 are from twenty to twenty-two slender, rounded, tole- 

 rably regular, longitudinal ribs, becoming evanescent 

 towards the suture, which is distinctly marked by a 

 fine line. The intervals between the ribs are in 

 general slightly wider than the ribs themselves, and 

 are shallow, flattish, and furnished with fine irregular 

 longitudinal wrinkles, especially on the first volution. 

 Both ribs and intervals are crossed by nine or ten fine 

 thread-like striae, producing a reticulated surface ; two Mitra juniperus. 

 of these are at the posterior part of the volution near 

 the suture ; in front of these is a smooth zone, equal to about one- 

 fifth of the exposed part of the volution, which is destitute of striae ; 

 the remaining striae are at the anterior portion of the volution. The 

 anterior portion of the first volution has in addition about twenty 

 more striations, regular and closely compacted, which are concealed 

 by the succeeding volution. Aperture narrow ; columella with three 

 strong, rounded spiral folds, the hindermost largest, the anterior one 

 slight, their intervals broad, flat and smooth ; between the two ante- 

 rior ones is a slight trace of another. The medial portion of the 

 outer lip is occupied interiorly by eleven or twelve raised thread- 

 like lines, which penetrate into the mouth as far as can be seen. 

 Length about 1*6 inch, breadth '45 inch, first volution '6 inch : 

 angle of spire 1 8°. 



On the Wealden Beds of Brora, Sutherlandshire, with Re- 

 marks on the Relations of the Wealden Strata and Stonesjield 

 Slate to the rest of the Jurassic System, and on the marine con- 

 temporary of the Wealden Series above the Portland Stone. By 

 Alexander Robertson, Esq., F.G.S. 



[Read May 20, 1846.] 



In the spring of 1843 I had the honour of transmitting to the 

 Society a short account of two beds containing freshwater fossils, 

 which were found by me, during the preceding summer, intercalated 

 with the carboniferous portion of the Jurassic series of Brora*. 

 Having been prevented at the time of the discovery from making so 

 detailed an examination of their relation to the rocks with which 

 they were associated as was desirable, I again visited the locality, and 

 now present the result of my observations on that occasion. 



My first object was to determine the exact age of the marine 

 bed immediately above the main seam of coal, and with this view I 



* Proc. Geol. Soc. vol. iv. p. 173. 

 Vol. Ill part i. i 



