520 Reviews — Fossil Insects from Commentry. 



YI. — Fossil Insects in Coal-measuees. 



Two Insects from Commentey. — R. J. Tillyard (Proc. Linn. Soc. 

 l^.S. Wales, xliii, pp. 123-134, March, 1918) discusses two fossils 

 recently described by H. Bolton (Manchester Memoirs, May, 1917). 

 He suggests that Megagnatha odonatiformis Eolton, is an ancient 

 representative of the Order Embioptera, and erects for it a new 

 family, Megagnathidse, differing in greater size and more complex 

 venation, as well as, probably, in the shorter comparative distance 

 between the bases of the fore- and hind-wings. Sycopteron 

 symmetricum Bolton, "is very likely an archaic type of the Order 

 Psocoptera, related to Amphientomum of the Oligocene, but con- 

 siderably less specialised" in its venation. 



VII. — RocK-BOEiNG Oeganisms AS Agents IN CoAST Erosion. By 

 Professor T. J. Jehu. Scottish Geographical Magazine, vol. xxxiv, 

 pp. 11, with 3 figures, January, 1918. 



IN this paper the author lays stress on the importance of the part 

 played by rock-boring organisms in submarine erosion, and more 

 particularly in the lowering of the foreshore, with the consequent 

 further exposure of the cliffs to the wearing action of the waves. 

 He shows how this process is carried on to a very great extent near 

 Cromer and Brighton, where the chalk forms the sea bottom and 

 foreshore, and also at St, Andrews, where the rocks are sandstone, 

 shale, and limestone. The rocks are perforated by the organisms, 

 with the result that they are converted into a honeycomb-like 

 network, which is easily broken down by mechanical agencies, at first 

 to an irregular surface, which is soon planed down to an even 

 platform at the new lower level. This destruction is not continuous, 

 as it can only take place where the rock is free from loose overlying 

 sediment, but in the absence of such sediment it proceeds in very 

 many places, and at an average rate of about 1 inch per annum. 



Tlie work is carried out by a great variety of forms, including 

 annelids, sponges, molluscs, and echinoderms; these creatures mostly 

 prefer soft or calcareous rocks, but some, more especially Pholas, 

 bore into anj^ kind of rock, including sandstone, mica-schist, and 

 gneiss. The means by which this animal bores the rocks are some- 

 what obscure and many different suggestions have been put forward. 

 These are summarized as follows : — 



1. That the perforations are made by rotations of the shell-valves, 

 after the manner of augers. 



2. That the holes are made by rasping, effected by siliceous 

 particles in the foot, or mantle, or both. 



3. That the excavations are due to ciliary currents, aided by 

 rasping. 



4. That the horing is carried out by the foot exerting suction. 



5. That the rasping is brought about by the friction of gritty 

 particles of external derivation against the walls of the burrows. 



The balance of evidence goes to show that, at any rate in the 

 case of the Pholadidse, the action is mechanical, though acid secretion 

 may play some part. In experiments carried out by Miss B. Lindsay 



