DR. T. ALCOCK ON POLYMORPHINA TUBULOSA. 247 



In order to follow the successive changes in the latter 

 part of the life of this Polymorphina, as they are illustrated 

 in the specimens before you^ the large rounded shells of 

 P. communis should be first noticed^ in which no opening 

 is perceptible excepting the mouthy showing that at this 

 stage the numerous large holes which are afterwards formed 

 have no existence. The great thickness of the outer walls, 

 compared with that of the internal parts of the shell, shows 

 that the animal must have existed for a considerable 

 time in this condition, during which the surface has been 

 strengthened by repeated deposits of calcareous matter 

 from its coating of external sarcode ; and the smoothness 

 and evenness of this surface shows that the coating was at 

 that time spread uniformly over the whole of it. But 

 broken specimens of P. tubulosa show that a change in 

 the disposition of the external sarcode has been afterwards 

 made ; for in these it is found to have collected itself into 

 two or three irregular bands, always commencing by one 

 end at the mouth and extending towards the base of the 

 shell — an arrangement clearly mapped out by the remains 

 of its ultimately formed shell covering, fragments of which 

 are seen still attached to the surface of the smooth rounded 

 nucleus. 



The next event in the life of this Polymorphina is the 

 formation of those numerous openings through the thick 

 shell-walls, the observation of which in the specimens 

 before you has chiefly led me to introduce them to your 

 notice. These show, by their definite position and the 

 evidence they give of their progressive formation, that, 

 when the external sarcode has once taken the form of 

 bands, it remains permanently in that state, and that these 

 bands hold a fixed position on the parts of the shell where 

 they were at first placed. Among the specimens shown 

 are some which only differ from ordinary shells of P. 

 communis in being Temarkably smooth on the surface, and 



