DR. T. ALCOCK ON POLYMORPHINA TUBULOSA. 245 



Williamson^s P. communis, and appears to be identical 

 with it, this form, so far as I have seen, only taking on 

 the peculiar final development characteristic of P. tubulosa. 

 It consists, in the mature state, of the rounded shell of P. 

 communis, more or less concealed by several covered pas- 

 sages, commencing at the mouth and taking a direction 

 towards the base of the shell ; these passages have their 

 arched walls developed into tubular prolongations, extend- 

 ing in all directions, and soon dividing irregularly into 

 small branches, which, in one or two instances in the 

 specimens shown, will be found to anastomose ; they are 

 either closed at their tips, as a small glass tube might be 

 closed in the flame of a blowpipe, or they expand into 

 little cauliflower-like excrescences, which are also apparently 

 closed. The shell composing the parts just described is 

 very delicate and thin compared with that forming the 

 rounded nucleus; and its outer surface is frosted with 

 small glassy projections of an irregularly squared figure, 

 like imperfectly formed crystals. It is evident that this 

 is a hastily deposited shell-covering on the sarcode, de- 

 veloped since the last regular chamber of the shell was 

 formed, and which, instead of collecting itself into a de- 

 finite shape, to produce a chamber similar to the others, 

 had been surprised, as it were, while fully expanded, 

 by the calcifying, process, which consequently gives 

 us a petrified representation of the ordinary appear- 

 ance of this external sarcode with its pseudopodia pro- 

 truded, the probable suddenness of the process being illus- 

 trated by the cauliflower excrescences which terminate 

 many of the branches, and which have resulted from the 

 contraction of the extremely fine terminal filaments of sar- 

 code. It would appear that this is the final act in the life 

 of the Polymorphina, its enfeebled vital power having been 

 insufficient to gather together the sarcode for the formation 

 of another regular chamber ; and therefore, properly speak- 



