148 H. L. Hawkins — Dental Apparatus of Discoidea. 



have originated by the collapse of the shell in the shrinking nncleus 

 of the globe, once these have been flattened out longitudinally by this 

 earth creep. 



The subject first suggested itself to me when working on the rocks 

 of Tristan da Cunha, and the map here reproduced is taken from my 

 paper on " Tbe Eocks of Tristan da Cunha brought back by H.M.S. 

 Odiu, 1904, and their bearing on the question of the Permanence of 

 Ocean Basins".^ Dr. Katzer, in his "A fauna devonica do rio 

 Maecuru, e as suas rela^oes com a fauna de outros terrenes devonicos 

 do giobo ",- di'ew his " Continente atlantico-aethiopico " on more or 

 less the same lines, but terminated it to the south by a shore running 

 latitudinally, parallel' with the southern extremity of South Africa, 

 deriving his sediments in the Falkland Islands from a " Continente 

 meridional ". The sediments in the Falkland Islands, however, are 

 so similar to those in the Bokkeveld Beds in South Africa that I am 

 inclined to think they must have been derived from the same continent. 

 The important point to notice is that Dr. Katzer, working from Brazil, 

 arrived at the same distribution of land and water in Devonian times 

 as I did workina; from the other side of the Atlantic. 



II. — Oisr THE Jaw Apparatus of Discoidea cylindeica (Lamaeck). 



By Herbert L. Hawkins, B.Sc, Mark Stirrup Scholar in the University of 



Manchester. 



(PLATE VI.) 



WHILST searching through the duplicate drawers in the Manchester 

 Museum I found recently a specimen of Discoidea cylindrica 

 which contained the dental apparatus in a very good state of preserva- 

 tion. The softness of the matrix within the test made it possible to 

 develop and extract the jaws, and thus to detect several new features 

 of interest. 



The jaws of Discoidea were described in 1892 by Loven, who gave 

 good figures of the maxillae both in the text (p. 53) and on plate x 

 (figs. 114, 115). In this paper he discusses the two distinct types 

 of perignathic girdle that occur in Discoidea cylindrica. He states 

 that in the depressed form {forma vulgaris^ the ambulacral processes 

 ('auricles') are low, and connected by interradial ridges ('walls') 

 which are of almost the same height ; whereas in the tall form {forma 

 elatior) the processes are much more prominent than the ridges. The 

 jaws he describes are from the form he designates as the " more 

 normal ", but it is hard to determine to which type he refers in this 

 phrase. There is no mention of the teeth or of the epiphyses in 

 Loven's paper, but both these structures are preserved in the specimen 

 about to be described. 



It will, perhaps, be best to proceed with the description of the 

 specimen, and subsequently to consider the differences from Loven's 

 description. 



The specimen, which is one of a pair of small DiscoidccB collected 



1 Trans. Phil. Soc. S.A., Cape Town, 1905, vol. xvi, p. 19. 

 * Boletin do Museu Paraense, 1895, vol. ii, Para, Brazil. 



