292 Dr. R. K. Greville on the discovery of 



Middle Silurian group ; and on the fossil evidence in the Cam- 

 bridge Museum Professor M^Coy ventured to affirm his con- 

 viction, that some great sectional or palseontological error had 

 been committed in the establishment of the middle group. 



We put this conclusion to the test in 1852 ; and on good 

 sectional and fossil evidence, we were enabled to break up this 

 group into two parts, which were not only palseontologically 

 distinct, but generally unconformable one to the other. Thus 

 the May Hill Sandstone became at length the true base (phy- 

 sically and palseontologically) of the whole Silurian series ; and 

 by the interpolation of that sandstone there will be no longer 

 any real difficulty in the tabular view which will precede the 

 Third Fasciculus*. 



I might here (as not by any means unconnected with the 

 subject of this communication) also dwell upon the great value 

 of Professor McCoy's determination of the Devonian corals, as 

 distinguished from the Cambrian, the Silurian, and the Carbo- 

 niferous : but I must forbear ; and I hope to take up some of 

 the subjects, here pointed at, in a future communication. 



XXVII. — Notice of the discovery of Desmarestia Dresnayi on the 

 coast of Ireland, By R. K. Greville, LL.D. &c.t 



[With a Plate.] 



The Alga to which this notice refers was collected towards the 

 close of last year, at Moville, near the mouth of Lough Fyle in 

 the north of Ireland, by William Sawers and — Morrison, Esqrs., 

 and communicated by the former gentleman to Professor Balfour, 

 by whom specimens were placed in my hands for examination. 

 It is a form quite new to the British flora ; and as its affinity is 

 involved in some obscurity, its discovery on our shores is in- 

 vested with considerable interest. Specimens transmitted to the 

 celebrated French cryptogamist. Dr. Montague, have been pro- 

 nounced by him to be identical with an Alga found by himself at 

 Fort St. Sebastian in 1823, and published in the 'Annales des 

 Sciences Naturelles^ for 1842, p. 251. t. 7. f. 2, under the name 

 of Desmarestia pinnatinervia. Dr. Montague obtained only a 

 single individual, scarcely more than 4 inches high, fully 2 inches 

 wide, and truncate ; being evidently an abnormal development. 

 M. Cronan has likewise met with it, though rarely, at Brest, and 

 regards it as a variety oi Desmarestia Dresnayi 'oi Lamouroux, 



* Memoir read before the Geological Society of London, Nov. 3, 1852. 

 t Read before the Botanical Society of Edinburgh, January 12, 1854. 



