1866.] YOUNG PLATYSOMUS. 301 



7. EcHiNONEus cYCLOsTOMus, Leske. 



The very wide geographical distribution of this echinoderm prepares 

 us to expect to find it in a fossil state. It seems, however, to have 

 been rare in the Miocene period, compared with Echinolampas lyco- 

 persicus and JSchizaster Scillce. 



Anguilla. The single example I obtained is very small. 



8. ScHizASTER SciLL^, Desmoulius. 



Anguilla. Rather common. It is also found in the Maltese beds. 



9. Brisstjs dimidiatus, Agassiz. 



Found in Anguilla. The fossil is, if I have correctly determined 

 it, identical with the species living in the adjoining seas. It is very 

 rare as a fossil at Anguilla. 



EXPLANATION OF PLATE XIX. figs. 4-8. 



{Illustrative of West-Indian Tertiary Echinoderms.) 



Figs. 4a, 46. Echinolampas ovum-serjpentis, Gruppy. Natural size. 



6. . A small example, showing the position of the vent 



and pentagonal mouth. Natural size. 

 6. . A circular form with ovate transverse mouth. Na- 

 tural size. 



7. semiorhis, Guppy. One-half the natural size. 



8. lycopersicus, Gruppy. Natural size. 



4. On the Afmnities of Platysomus and allied Genera. By John 

 Young, M.D., F.G.S., of the Geological Survey of Great Britain. 



(Communicated by Professor Huxley.) 



[Plates XX. & XXI.] 



The genus Platysomus, as defined by Agassiz, and placed by him 

 among the heterocercal Lepidoids with flat broad bodies, contains 

 forms which cannot be included under the definition he has given. 

 The imperfect materials from which his descriptions were drawn 

 up have since been supplemented by abundant well-preserved speci- 

 mens, whence it is comparatively easy to fill up the masterly outline 

 he has sketched in the ' Poissons Fossiles.' 



The first change in the classification of the group to which Platy- 

 somus belongs was proposed by Sir P. G. Egerton (Quart. Journ. 

 Geol. Soc. v.). The occurrence in a mandible of PI. macrurus, Ag., 

 of teeth with a blunt conical crown and constricted neck, and the 

 discovery of the true nature of the apophyses (compared by Agassiz 

 to the Y-shaped bones of Clupea), suggested the affinity of that 

 species to the Pycnodonts, to which family, therefore. Sir Philip re- 

 ferred the whole genus, and along with it Globulodus, Miinst., 

 whose relation to Platysomus, Agassiz had previously suspected. 

 This change Agassiz accepted. But the '* dents en brosse " mentioned 



