OF THE PROTOZOA. RHIZOPODA. 225 



universa and in Acei'vuUna globosa (XXI. 35-37) they obtain their greatest 

 diameter. In the latter, and in Glohiger'ina, the canals dilate towards the 

 sui'face, and are consequently fimnel-shaijed (infimdibidiform). In a few 

 instances two different sorts of pores exist, as in Orbulhm universa and 

 Eosalina varicnis, the finer kind being more abundant. 



A peculiar sort of slits is characteristic of the genus Pohjstomella ; that they 

 completely perforate the shell is shown by sections. They are largest in P. 

 strigilaUi, and in P. gihha apj^ear to be only shallow excavations. 



Besides the openings named, the surface of the shells often presents regularly- 

 disposed eminences or elevated lines. In Pohjstomella strigilata and P. venusta 

 (XXI. 28-30) there are hemispherical or conical eminences, perforated severally 

 by a fine opening. In Textilaria picta elevated lines are arranged around the 

 widely-separated pores, so as to produce an elegant design (XXI. 2b). Lastly, 

 many shells have a spinous or stellate appearance, from the prolongation of some 

 canals into long and fine projecting tubes, or from that of the whole of them 

 into thick processes. Illustrations are afibrded by Rosaluia Imperatoi^ia, Cal- 

 carina, and particularly by Siderolina calcitrapoides. 



Carter has described a greenish, perishable, organic membrane as investing 

 the entii^e suiface of the shells ^\dth all their irregularities ; and d'Ai-chiac 

 has assumed this to be the secreting membrane of the calcareous matter. 

 Schultze, however, has failed to detect such a structure in every specimen he 

 has examined, whether in a Hving or in a dried condition ; and he observes 

 that, even if this membrane does exist in certain cases, there are abundant 

 facts to prove that it is not the seeretiug organ of the sheU. 



The foramina are, as a i-ule, uniformly distributed over the shells, those 

 parts only being free which are placed immediately above the partitions 

 between adjoining cells. Exceptions, however, occur. Thus, in the iuequi- 

 lateral Rotalidce (XXI. 33) and their allies, the under or umbilical side has 

 fewer pores than the upper. Also, iu some of the thick-shelled species the 

 position of the subjacent septa are not indicated by the absence of pores. 

 The long winding canals pass in difterent directions, unite, and appear on the 

 surface in groups, producing a complex wavy pattern on the surface, as in 

 many Calcariiue. 



The partitions between the several cells are perforated by oiifices, which 

 differ in size, number, and distribution in the several species. They occur in 

 the septa as fine pores similar to those of the surface, but in less number. 

 Again, in species having a single large opening in their terminal chamber, 

 there is a similar one in each partition, as in Nodosarida, Miliolkla (XXI. 21, 

 22), Textilaria (XXI. 36), Rotalida, and in Nonionina, Rotidina, Cristellaria, 

 &c. Among this group the Comdina form an exception, in having numerous 

 foramina in the last cell and in the septa between the others. In Acervulina, 

 again, the several cells communicate by a single opening. In Peneroplis, 

 Cosciiiospira, and in Pohjstomella the septa have numerous pores ; and the 

 foramina proportionally increase in number mth the increasing size of the 

 septa, i. e. from the fii\st- to the last-formed chamber (XXI. 28-30). In 

 Orhicidina the thick septa are penetrated by canals. 



Ehrenberg pointed out the presence, in several species, of numerous per- 

 pendicidar calcareous columns interposed between the septa, which he sup- 

 posed to be hollow tubes, opening up a communication between the whole 

 series of chambers and the exterior. Both their fimction and their tubular 

 natui-e Schultze disbeheved, and asserted that Lunulites (Etw.) is not one of 

 the Polythalamia, but actually a colony of Bryozoa. 



Mr. Carter {A. N. H. 1852, x, p. 170), on the contrary, asserts the ex- 

 istence of such tubes in the septa, in the following passage : — 



Q 



