FOKAMINIFERA 8oi 



that whilst the aperture of communication between the chambers 

 and between the last chamber and the exterior is usually very small 

 in the ' vitreous ' shells, serving merely to give passage to a slender 

 stolon or thread of sarcode from which the succeeding segment may 

 be budded off, it is much wider in the ' porcellanous' shells, so as to 

 give passage to a ' stolon ' that may not only bud off new segments, 

 but may serve as the medium for transmitting nutrient material 

 from the outer to the inner chambers. 



Between the highest types of the porcellanous and the vitreous 

 series respectively, which frequently bear a close resemblance to 

 each other in /orm, there are certain other well-marked differences 

 in structure, which clearly indicate their essential dissimilarity. 

 Thus, for example, if we compare Orbitolites (fig. 609) with Cyclo- 

 clypeus we recognise the same plan of growth in each, the chamber- 

 lets being arranged in concentric rings around the primordial 

 chamber ; and to a superficial observer there would appear little 

 difference between them. But a minuter examination shows that 

 not only is the texture of the shell ' porcellanous ' and non-tubular 

 in Orbitolites, whilst it is ' vitreous ' and minutely tubular in Cyclo- 

 dypeus, but that the partitions between the chamberlets are single 

 in the former, whilst they are double in the latter, each segment of 

 the sarcode-body having its own proper shelly investment. More- 

 over, betwen these double partitions an additional deposit of cal- 

 careous substance is very commonly found, constituting what may 

 be termed the intermediate skeleton ; and this is traversed by a 

 peculiar system of inosculating canals, which pass around the 

 chamberlets in interspaces left between the two laminae of their par- 

 titions, and which seem to convey through its substance extensions 

 of the sarcode-body whose segments occupy the chamberlets. We 

 occasionally find this * intermediate skeleton ' extending itself into 

 peculiar outgrowths, which have no direct relation to the chambered 

 shell. Of this we have a very curious example in Calcarina ; and it 

 is in these that we find the 'canal system' attaining its greatest 

 development. Its most regular distribution, however, is seen in 

 Polystomella and in Operculina ; and an account of it will be given 

 in the description of those types. 



Porcellanea. Commencing, now, with the porcellanous series, 

 we shall briefly notice some of its most important forms, which are 

 so related to each other as to constitute but the one family Miliolida. 

 Its simplest type is presented by the Cornuspira of our own coasts, 

 found attached to seaweeds and zoophytes ; this is a minute spiral 

 shell, of which the interior forms a continuous tube not divided into 

 chambers ; the latter portion of the spire is often very much flattened 

 out, as in Peneroplis, so that the form of the mouth is changed from 

 a circle to a long narrow slit. Among the commonest of the Fora- 

 minifera, and abounding near the shores of almost every sea, are 

 some forms of the milioline type, so named from the resemblance of 

 some of their minute fossilised forms (of which enormous beds of 

 limestone in the neighbourhood of Paris are almost entirely com- 

 posed) to millet-seeds. The peculiar mode of growth by which these 

 are characterised will be best understood by examining, in the first 



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