Bev. T. G. Bonney — Burrows in Limestone Rock. 483 



the non-continuance of the numerous ridges down the sides. Hyhodus 

 formosus diifers in the larger ridges, with wider intervals, and a more 

 regular disposition over the sides of the spine. In both H. reticu- 

 latus and H. formosus, the halves of the hinder border meet at a less 

 open angle, and the median dentated ridge is more prominent; con- 

 sequently the hind border meets the lateral surface at a more open 

 angle than in IT. complanatus. 



From the known species of Acrodus, the present specimen differs 

 in the pattern of the ridging of the sides : the ridges in Acrodus are 

 fine, but regularly longitudinal, and in some specimens almost 

 obsolete. The sides of the enamelled body of the spine are less flat, 

 more convex, in Acrodus, and the spine is thicker in proportion to 

 the breadth in Acrodus than in Hyhodus. 



The length of the present spine of S. complanatus, is 1 foot 4 inches ; 

 the breadth is 1 inch 9 lines ; the thickness posteriorly is nearly 

 1 inch. 



The proportions of the enamelled and unenamelled surfaces of the 

 spine, the obliquity of the terminal basal line of the ganoin, and the 

 excavation of the smooth implanted basal part or root, accord with 

 the usual Hybodont type of Ichthyodorulite. 



II. — On the Supposed Occurrence of Pholas Burrows in the 

 Upper Parts of the Great and Little Ormesheads. 



By T. G. Bonney, M.A., F.G.S. 

 (PLATE XVII.) 



IN a paper entitled " On Traces of Grlacial Action near Llandudno," 

 printed in the Gteological Magazine, Vol. IV. p. 289, I des- 

 cribed certain indications of glacial action (as they appeared to me) 

 upon the two masses of Carboniferous Limestone, well known to 

 visitors at Llandudno under the naine of the Great and Little Ormes- 

 heads. The principal conclusion at which I arrived was that, " after 

 the limestone hills of the district had acquired their leading forms by 

 upheaval and marine denudation, the whole district was depressed. 

 The summits of the low rocky islets thus formed became capped 

 with ice-fields, which in places descended in glaciers into the sea. 

 At times, very probably, they were united to the mainland by pack 

 or coast ice." I then supposed that, after some minor changes, " the 

 whole was gradually lapheaved above the sea, probably — at any rate, 

 in the case of the Great Ormeshead — not quite uniformly." 



This conclusion was opposed by Mr. E. D. Darbishire in a paper 

 read before the Literary and Philosophical Society of Manchester, 

 and published in their Memoirs (ser. 3, vol. iv.), a copy of which he 

 very kindly sent to me. In this, after mentioning sundry indications 

 of marine action, he describes certain burrows which he has discovered 

 on various parts of the above hills, and considers to have been 

 formed by a species of Pholas, probably P. crispata. He comes, 

 therefore, to the following conclusion : "In the preservation of these 

 burrows, in loose beach stones, and in the edges of the tables of out- 



