Prof. H. A. Nicholson — On Lower Silurian Chcetetes. 175 



distinction noticed in the north holds good — comparative abundance 

 in the Gravels, and greater rarity in the Boulder-clay. It appears as 

 well that they have been seen in the gravels of Wexford and of 

 Cork. 1 



It must be borne in mind that the Irish Chalk is of a very hard 

 texture, almost as hard as ordinary Limestone, and much harder than 

 the Coal-measure Shales, Bunter Sandstone, and Keuper Marls, all 

 of which occur so plentifully in the "Till" of the northern district; 

 so that its absence cannot be ascribed to crushing and pulverization 

 by the ice-sheet, nor can it be laid to the score of solution by water 

 containing carbonic acid, for this would also affect the Limestone. 

 Besides, neither of these suppositions would account for the scarcity 

 of Chalk-flints. 



VI. — On Some of the Massive Forms of Ch^tetes, from the 

 Lower Silurian. 



By H. Alleyne Nicholson, M.D., D.Sc, F.E.S.E., 

 Professor of Biology in the College of Physical Science, Newcastle-on-Tyne. 



Ch^tetes petropolitanus, Pander. 



The typical forms of Chcetetes petropolitanus, Pander, are free in 

 habit, and are invariably furnished with a flattened or concave base, 

 which is covered with a thin concentrically- wrinkled epitheca. The 

 general shape of the corallum is typically a cone or hemisphere 

 when full-grown, and a concavo-convex disc when young ; but its 

 adult form is liable to great variations ; and even the young examples 

 are not constant, being sometimes plano-convex, or slightly bi-convex. 

 The calices are exclusively carried upon the upper surface of the 

 mass, and are thin-walled, polygonal in shape, and destitute of inter- 

 mediate tubuli. Very often the surface exhibits minute tubercles, 

 carrying corallites of a slightly larger size than the average ; but 

 this phenomenon does not appear to be constant. Young examples 

 may have a diameter at their base of four or five lines, and a height 

 of three lines ; adult specimens may have a long diameter of four 

 inches or more at the base, and a height of an inch and a half ; but 

 the majority of individuals exhibit dimensions intermediate between 

 these measurements. 



Stenopora patula, Billings (Canadian Naturalist, vol. iv. p. 427), 

 as hinted by Mr. Billings himself, is to be regarded as only a well- 

 marked variety of C. petropolitanus. It agrees with the typical 

 examples of the latter in its free habit, in its surface-characters, and 

 in its possession of a concentrically-wrinkled epitheca. It is 

 peculiar only in its shape, having the form of an approximately 

 circular disc, the basal surface of which is flat, or slightly con- 

 cave, and is covered by an epitheca ; whilst the upper surface is 



got two perfect shells. One a Purpura Inpillus, the other a small bivalve which 

 I have not been able to identify with certainty, but it appears to be Tellina solidula. 

 i On Ee-arranged or Glacialoid Drift, by G-. H. Kinahan, M.E.I.A., Geological 

 Magazine, Decade II. Vol. I. No. 3, pp. 112 and 119. 



