52 BRITISH PALEOZOIC ASTEROZOA. 



the Olio liand, to forms witli completely alternate arabulacral ossicles, and on the 

 other to those in which these ossicles are opposite, at first free, afterwards having 

 slight connections, and increasing in vertical extent, and that in both gronps 

 the buccal armatnre has advanced in complexity " (71, p. 223). 



There can be no doubt that these authors are correct in their main assumptions, 

 namely : 



(1) That the ])eculiarities in structure of these old forms are not of sufficient 

 magnitnde to allow us to neglect their real Ophiuroid affinities. 



(2) That there has been a double line of progression within the group itself. 

 If, however, as is granted by Professor and Miss Sollas, there is a double line 



of progression, we must ask ourselves, Are we entitled to place all the forms in one 

 group ? There appears to be no doubt that the resemblances between the 

 " Ophiuroidea," with opposite and alternating ambulacralia, are due to the fact 

 that there has been parallel (homoplastic) evolution in the two sets of forms. 

 The number of divergent branches of the Asterozoan stock cannot be expressed b}^ 

 the present dual division into Asteroidea and Ophiuroidea — a classification which, it 

 nuist be remembered, is merely based on knowledge of the Recent survivors of man}- 

 ages of experiment and trial. 



I have only bad space in this Introduction to describe a few of the medley of 

 specialised forms found in the older Palaeozoic rocks. Some of the families into which 

 the species can be grouped are far from easy to fit in grouping which is familiar to 

 us at present. It therefore appears desirable that a detailed description of the 

 material at hand should be given before a further new classification is attempted. 



LITERATURE. 



1. Baily, W. H., " Explanatory Memoir to accompany Sheet 34 of the map of 



the Geological Survey of Ireland," 'Mem. Geol. Survey,' 1878. 



2. 'Explanatory Memoir to accompany Sheets 1G9, 170, 180, and 181 of 



the map of the Geological Survey of Ireland,' 187*.). 



3. Bather, F. A., " Studies in Edrioasteroidea," ' Geol. Mag.' [4], vols, v, vi, 



[5], vol. V, 1898—1908. 



4. " AVhat is an Echinoderm? " ' Journ. London Coll. Sci. Soc.,' vol. viii, 



pp. 21—33, 1901. 



5. ■ " Sympterura minveri, n.g. et sp. : A Devonian Ophiurid from Cornwall," 



'Geol. Mag.' [5], vol. ii, pp. 101— 1G9, London, 1905. 



6. — " Australian Palfeontologists on Silurian Ophiuroids," ' Geol. Mag.' 



[5], vol. iv, pp. 313, 314, 1907. 



7. 'A Treatise on Zoology,' edited by E. Ray Lankester, vol. iii, " Echiuo- 



derma," 1900. 



8. " Triassic Echinoderms of Bakon}^" ' Result, wissensch. Erforsch. 



Balatonsees,' Bd. i, Pal. Anhaug, Budapest, 1909. 



