Sir H. Hou'orth — Recent Changes of Level. 257 



of tlie bases of the two spines, the portion belonging to each being 

 in one of my specimens apparently divided from the other by a 

 suture ; though in others no such division can be seen. 



As regards Isclmacanthus, it seems to me that no one can look at 

 its shoulder without being convinced that the ossicle supporting its 

 pectoral spine is perfectly homologous with the corresponding 

 element in Climatius, Eiith acanthus, Parexus and Diplacanthns on 

 the one hand, and Mesacantlms, Cheiracanthus and Acanthodes on 

 the other. Its upper extremity has become more cylindrical, its 

 lower is still considerably expanded. 



The shoulder-bone of Mesacantlms is still to some extent claviculoid 

 in appearance, the lower half having a laminar expansion, but in 

 Cheiracanthus and Acanthodes it has become almost cpn'te cylindrical, 

 save just at the lower extremity, where it joins the pectoral spine. 

 Here I may repeat that the relation of this element to the pectoral 

 spine all through the series of genera is so identical that for my 

 part I cannot conceive that the bone should be in one case a "basal," 

 in the other a "clavicle." Therefore I can also see no reason why 

 Ischiacanthus should be separated and put into a family apart from 

 the other Diplacanthid^. 



But what is the shoulder-bone after all? It cannot well be a 

 basal bone, as microscopically it is, according to Keis, like the 

 spines and scales, entirely composed of Dentine,^ and its configura- 

 tion, and association with another plate-like " infra-clavicular " 

 element in certain Diplacanthidas, pretty certainly indicate that it 

 is also, like the spines, entirely superficial in its origin. I therefore 

 must concur with Reis in considering this bone to be a " claviculoid," 

 or a dermal structure assuming the position and functions of a 

 clavicle. 



V. — The Most Eeobnt Changes of Level and their Teaching. 

 Part I. The Raised Beaches. 



By Sir Henry H. Howorth, K.C.I.E., M.P., F.R.S., F.G.S. 



THERE are signs accumulating everywhere that the views so 

 logically pressed to their conclusion by Hutton and Playfair, and 

 by a great catena of geologists, since the appearance of the first 

 edition of Lyell's " Principles of Geology," have received a certain 

 check ; and no one can read the works of the great Continental 

 geologists without seeing that there is a tendency to reconsider 

 the position, and to hark back to the views of another school of 

 teachers. 



While conceding that diurnal and slowly operating causes have 

 done much to smooth and shape the superficial features of the 

 world, many sober geologists are beginning to realize that it is to 



^ In the substance of this bone there seem to be no vascular canals. Eeis (Zur 

 Kenntnias des Skelets der Acanthodinen. Geognostische Jahrbucher, 1890, pp. 27 

 et seq.) describes and figures dentine tubules, but these were not observed by Fritsch, 

 who describes the structure as consisting entirely of closely superimposed laminae 

 (Fauna der Gaskohle und der Kalksteine der Permformation Bohmens, Bd. iii. 

 Heft 2, pp. 54, 55). 



DECADE IT. VOL. I. NO. VI. 17 



