B. Milne Home — Glaciation of the Shef lands. 449 



species the thec^ are seen to be separated from the perisarc 

 by a distiuctly-raarked septum. The perisarc is, moreover, in 

 specimens of all the three species, seen to be jointed, or crossed 

 by transverse septa, there being one septum to each theca. The 

 appearance is therefore that of a common perisarc divided into 

 chambers, from each of which a single isolated hydrotheca is pro- 

 duced. These appearances are not confined to the graptolites of the 

 Skiddaw Slates, having been noticed in well-preserved specimens 

 from the Lndlow Rocks, and, though not so clearly, in specimens 

 from the Lower Silurian Rocks of the South of Scotland. 



I have been led to adopt the following conclusions : — that it is 

 owing to the imperfect state of preservation in which graptolites 

 usually occur that the septa are not more frequently seen ; that 

 the true interpretation of the appearances presented is that the septa, 

 which seem to completely cut off the hydrotheca from the perisarc, 

 and the sections of the perisarc from each other, only partially do so, 

 as in the recent Thecaphora ; and that these specimens show that 

 the calycles of the graptolites are true hydrothecse, and do not in 

 any way invalidate the conclusion arrived at from previous investi- 

 gations into the morphology of the Rhabdophora that they are the 

 Paleozoic representatives of the recent Hydroida. 



In Mr. Dover's collection there are also many branches of Didymo- 

 grapti and Tetragrapti about a foot in length, some showing no signs 

 of termination at either end. 



VI. — The Glaciation of the Shetlands.^ 

 By David Milne Home, F.R.S.E., F.G.S. 



I HAVE read with interest, the reply by Messrs. Peach and 

 Home in the August Number of the Geological Magazine, to 

 my criticisms on their theory, that the glaciation of the Shetlauds is 

 due to a mer de glace from Scandinavia. 



Will you kindly allow to me space in your Magazine, to acknow- 

 ledge the courteous terms in which these gentlemen have discussed my 

 criticisms, and to notice, for the information of your readers, the three 

 points on which, as it appears to me, the controversy finally turns. 



I. I demurred to the remarkable statement, that, when this gigantic 

 mer de glace "abutted" on the Shetlands, coming from the N.E., it 

 "sioung" or "veered'' round to the N.W. and N.N.W., thus changing 

 its course into one more than a right angle to its previous course. 



The explanation of this phenomenon, given by Messrs. Peach and 

 Home, was, that thereby " it would follow the path of least resist- 

 ance " (p. 809). 



I answered, that if a mass of ice, alleged to be 6000 feet thick 

 and 200 miles wide, could be deflected from its course, by this small 

 cluster of islands, into a new path, viz. "the path of least resistance," 

 that path must have been " along the eastern seaboard of the group, 

 in a direction S. by W., and not across the islands in a N.W. 

 direction." 



^ Being rejoinder to Messrs. Peach and Home's reply to Mr. Milne Home's 

 criticisms. 



DECADE II. — VOL. YIII.— NO. X. 29 



