Prof. M. Schultze on Polytrema miniaceum. 413 



which is not unimportant for the determination of the species. 

 Thus, for example, Acervulina acinosa is very clearly distinguished 

 from Polytrema miniaceum by the greater diameter of its canals, 

 which usually measure 0'012 mill. 



If the calcareous matter be dissolved by dilute muriatic acid 

 from specimens which have been preserved in spirits, the organic 

 substance occupying the chambers is obtained free, forming a 

 true and connected cast of the internal system of cavities. From 

 these casts it can be proved, much better than from sections 

 which never open out more than one plane, that the interior of 

 the calcareous shell is divided into chambers (figs. 5, 6, & 7), 

 which are connected together by siphons. Especially at the base 

 and in the centre of the Polytrema the siphons are very sharply 

 divided from the chambers, whilst towards the surface the siphons 

 are frequently so dilated that they attain the diameter of the 

 cavities of the chambers, as is shown in the representation of 

 the cast of a small portion of the inner space of the rind of a 

 Polytrema in fig. 4. Although no regularity is to be detected 

 in the general arrangement of the chambers, the portion of a 

 natural cast of Polytrema shown by me in fig. 6 deserves especial 

 consideration. This was brought to light by breaking up a pi-e- 

 paration treated with muriatic acid. The perfectly irregular 

 contents of the superficial chambers having been removed, an 

 unmistakably spiral arrangement in the more deeply seated 

 chambers made its appearance. The connexion with the rest of 

 the mass was broken; so that only the six chambers figured could 

 be seen in their natural connexion. The finer structure of the 

 membrane and contents of these segments left no doubt that the 

 regularly grouped masses in figure 6 and those represented in 

 figures 5 and 7 really belonged to the same specimen. This 

 discovery agrees with that described by Carpenter in Tino- 

 porus^. 



The organic substance remaining after the treatment of speci- 

 mens of Polytrema preserved in spirits consists of an external 

 membrane and a tenacious brownish-red substance, rich in 

 strongly refractive granules and drops, which render it opake. 

 Both the envelope and contents precisely resemble the brownish- 

 red contents of the chambers of many Polythalamia. I have 

 figured this in plate 3. figs. 11 & 12, plate 5. figs. 12 & 13, and 

 in other parts of my work on the Organization of the Poly- 

 thalamia, and indeed partly from spirit-specimens, so that the 

 figui'es cited are directly comparable with those here given of 

 Polytrema. The organic envelope of the contents of the cham- 

 bers of the Polythalamia is described by me as follows, at p. 15 



* Phil. Trans. 1860, pi. 21. fig. 11, and pi. 22. figs. 2, 3 & 4. 



