198 Proceedings of the Royal Physical Society. 



Fig. 2. Cheirodota (?) Rohertsoni, a single well-preserved plate showing 

 the simple plain margin, triangular perforations, straight spokes, and central 

 space. Capelrig, near East Kilbride. Collection — D. Robertson, Esq., F.G.S. 



Fig. 3. Cheirodota (?) primceva, a plate with eight perforations round the 

 margin. Kinneil (R. Avon). 



Fig. 4. Reverse of another example, having eight perforations. Kinneil 

 (R. Avon). 



Fig. 5. Another plate of C. {?)primceva in which there are ten perforations. 

 Kinneil (R. Avon). 



Fig. 6. Another individual of the same exhibition, twelve perforating 

 holes. Kinneil (R. Avon). 



Fig. 7. The central non-jierforated space of a twelve-perforated plate. 

 Kinneil (R. Avon). 



Fig. 8. A plate with ten perforations, having a portion of the outer undu- 

 lated margin broken away. Kinneil (R. Avon). 



Fig. 9. Spicules of Cucumaria Hyndmanni (Forbes), after Diiben and 

 Koren (K. Svenska Akad. Handlingar, 1844, t. 4, f. 13, pars). 



Fig. 10. Plate and anchor of Sijna'pta, after Diiben and Koren {loc. cit., 

 t. 5, f. 59). 



Fig. 11. Calcareous pieces from integument of Cucumaria frondosa (Lin- 

 npeus), after Diiben and Koren {loc. cit., t. 4, f. 1). 



Fig. 12. Plate of Thyonidium pellucidum (Vahl), after Diiben and Koren 

 {loc. cit., t. 4, f. 17). 



Fig. 13. Plate of Thyoiie fusus (Miiller), after Duben and Koren {loc, cit., 

 t. 4, f. 41). 



PI. v.. Figs. 1-12, and PI. VI., Figs. 1-8, are all magnified about forty-five 

 times the original size of the fossils. 



N.B. — The originals of all the figures are in the cabinet of Mr James Bennie, 

 except that of PI. VI., Fig. 2, which is in Mr David Robertson's Collection. 



XI. Migratio7i of Mammals. By Professor Duns, D.D., 

 F.E.S.E. 



(Read 16th March 1881.) 



Abstract. — Having stated that under this familiar title 

 some of the most difficult questions in natural history fall to 

 be discussed, the author introduced his paper by a series of 

 queries, in which he indicated the aspects of migration, in re- 

 gard to which we have yet much to learn. It was shown that, 

 in attempting to answer these and kindred questions, natural 

 science often steps into the sphere of metaphysics, unaware, 

 the while, that in crossing the threshold it has parted with 

 its distinctive character. The relations between migration 

 and the geographical distribution of mammals were pointed 

 out. The a priori method had been too much followed by 

 students of the latter. Its weakness was shown, for 



