﻿The Development of Polypterus. 



75 



large occipital arches are present between metotic myotomes 

 I. and II., and that the skeleton of the pectoral fin is in the 

 earliest stage investigated in the form of a continuous plate 

 of cartilage. 



Central Nervous System. — In addition to points mentioned 

 in my previous communication/ it should be noted that in 

 the adult Polypterus there is a well-developed cerebellum 

 which projects forwards as a valvula cerehelli as in 

 Teleosts, but, in addition, projects backwards in a quite 

 similar manner into the cavity of the fourth ventricle. No 

 cerebral hemispheres are pushed out from the sides of the 

 primitive forebrain, the material which would form them 

 being accommodated by (1) thickening of the wall of the 

 primitive forebrain, (2) increase of its length, and (3) 

 slight invagination into the third ventricle. The conditions 

 in Polypterus support the view that the so-called "mem- 

 branous pallium " of the Teleostomatous fishes is simply the 

 roof of the Thalamencephalon, and is not actually homologous 

 with the " pallium " of higher forms. 



IV. Meristic Variation in the Common Sun-Star (Solaster 

 papposus). By D. C. M'Intosh, M.A., B.Sc, F.R.S.E. 



(Read 17th December 1906.) 



The purpose of this ]N"ote is to record the variation in the 

 number of arms in the Common Sun-Star [Solaster paj^posits) 

 as found in an examination of 612 specimens collected in 

 the Clyde area at intervals during 1902, 1903, and 1904. 

 I'he material examined was dredged by the S.Y. " Mermaid," 

 belonging to the West of Scotland Marine Biological Associa- 

 tion, and it was while occupying, through the kindness of 

 Professor J. Arthur Thomson, the Aberdeen University Table 

 at the Millport Station, that I gathered the facts herein 

 recorded. Solaster papposus, though widely distributed, is 

 not exactly a common animal in these waters. When 

 dredging, one rarely takes a haul without getting a few 

 specimens, but generally these number less than half a 



1 Proc. Roij. Phys. Soc, Yol. XVI. p. 191. 



