384 Dr. W. B. Carpenter on the Poljtremata. 



trema seems to consist in the grouping of these chambers 

 round large canals, which traverse the stem and branches, and 

 open at the extremities of the latter. Sometimes, however, 

 Polyh-ema spreads itself out peripherally, without any branch- 

 ing, so as to form subconical expansions, only distinguishable 

 externally from the outspread sessile forms of Tinoporus by 

 the opening of canals at or near their apices ; and in other 

 instances it forms compact globose masses, only distinguish- 

 able externally by their sessile habit, and by the presence of 

 canal-openings, from the ordinary globose forms of Tinoporus. 

 The closest resemblance to Garpenteria is presented by that 

 modification of Polytrema which is designated by Mr. Carter 

 as P. utriculare ; for in this we find large spreading cavities 

 taking the place of the canals, and opening externally by 

 prominent vents which bear a strong resemblance to those of 

 Garpenteria. But, like the canals of the branching P. mini- 

 aceum^ these cavities do not (as it seems to me) form any part 

 of the chamber-system, but are simply interspaces left in the 

 midst of what would otherwise be (as in Tinoporus) a con- 

 tinuous pile of minute chambers resembling those of the 

 original planorbuline base. 



On the other hand, as I stated in my memoir of 1860, the 

 arrangement of the primary chambers of the typical Garpen- 

 teria is distinctly spiral — the chambers all opening into the 

 depressed umbilicus, as in Ohhigerina^. This plan is clearly 

 traceable through the entire growth of the organism, — the 

 successive whorls spreading out by the rapid enlargement of 

 the chambers, and each whorl enclosing its predecessor ; so 

 that,thebase being progressively extended with the augmenting 

 height, a cone is built up, having a prominent apex in place 

 of the original depressed umbilicus. At the summit of this 

 cone there is always an apical orifice (sometimes prolonged 



**'I have fortunately been enabled to determine this point by the com- 

 parison of several specimens in dift'erent stages of evolution, and by 

 the removal from older specimens of one w^horl after another until the 

 original nucleus was arrived at (an operation which has been very 

 dexterously performed for me by my draughtsman, Mr. George West) ; 

 and I can state without any hesitation that the early condition of this 

 apparently anomalous organism accords with that of the Helicostegiie 

 Foraminifera generally, — its approximation being the closest to Rotalia 

 in its o-eneral form, but its tendency being rather towards Glohigerina 

 in this particular, that its chambers do not seem to communicate directly 

 with each other, but that each has a separate external oritice directed 

 towards the mnbilicus." (Phil. Trans. I860, p. 507.) Unless Mr. 

 Carter, by the dissection of a typical specimen of Carpenteria (such as 

 one of those on Mr. Cuming's Pontes) can show that the above de- 

 scription is erroneous, I must take leave to maintain its title to stand, 

 against his account of a supposed embryo of his Polytrema halanifonne. 



