886 THE ANIMAL KINGDOM. 



varying directions, but inclosed by a thick and finely perforated calcareous 

 mass ; or it becomes irregular, and this is especially the case when it is 

 attached, e. g. in the imperforate Nubecularia, the perforate Carpenteria, 

 where in C. raphidodendron it is arborescent. But in most instances of 

 this kind growth is at first regular. Exceptions to this rule however occur 

 among the Rotalid Tinoporinae, e. g. in the encrusting or arborescent 

 Polytrema, or in the Lagenid Raimilina, with chambers connected to one 

 another by long tubes. The single-chambered test may be more or less 

 spherical, as in the imperforate Squamula, the perforate Orbtilina, vasi- 

 form, as in the perforate Lagena, or piano-spiral, as in the imperforate 

 Cornuspira and the perforate Spirillina. When the test is polythalamous, 

 its chambers may follow one another in a more or less straight or arcuate 

 and single series, e. g. Nodosaria, seldom in a bi- or tri-serial series, as is 

 sometimes the case in Polymorphism, or they are disposed in a spiral with 

 the elements lying either in the same plane, e. g. in Biloculina, the nautiloid 

 Polystomellinae and Ntimmulites, or in a more or less helicoid or trochoid 

 manner as is generally the case. Successive chambers or coils may inclose 

 their predecessors, i. e. the shell becomes involute, e. g. in the imperforate 

 Biloculina and the perforate Hastigerina J . Spiral growth may give way 

 to a cyclical, due to the excessive widening in a planospiral test of the 

 peripheral chambers, which meet sooner or later round the first formed 

 portion of the shell, as in some species of Orbiculina and the genus Orbi- 

 tolites, among imperforate forms, and the Cycloclypeinae among perforate 2 . 

 Or the axis of convolution may be lengthened out, as in the Alveolinae 

 and Fusulininae respectively, in the imperforate and perforate series, the 

 chambers consequently attaining a great width and the shell a fusiform 



1 The term ' spiral axis ' applied to a coiled test denotes the longitudinal axis of the convo- 

 lutions ; ' plane of convolution ' a vertical plane coiled like the test ; ' axis of convolution ' a 

 line drawn through the point at which the test commences, and round which it is imagined to be 

 coiled. See on the subject of the spiral coiling, von Moller, Mem. Imp. Acad. St. Petersburg (7), 

 xxv. No. 9, pp. 27-40. The perforate family Cheilostomellidae is remarkable for the degree to which 

 involution is carried. In the extinct Ellipsoidina every new chamber completely incloses its pre- 

 decessors, all being connected at their bases ; i. e. the test resembles a number of flasks standing one 

 within the other. So, too, in Cheilostomella, but every new chamber has its aperture turned in the 

 opposite direction to its immediate predecessor ; whilst in Allomorphina the chambers are disposed 

 in cycles of three, the last chamber leaving a portion of its two predecessors exposed. 



2 The cyclical Orbitolites has a very perfect series of species showing its evolution from Orbi- 

 culina, which is itself derived from Peneroplis. The Orbiculine portion of the test is extremely 

 large in O. tenuissima, and is very decidedly excentric, features very much less marked in 0. margi- 

 nalis, whilst in 0. duplex and 0. complanata the excentricity is lost, and the cyclical mode of growth 

 established almost from the first, the last named being the most advanced. There is also in the two 

 last named species a complication in the structure of the chamberlets, 0. duplex leading from the 

 simple type of 0. marginalis to the complicated type of 0. complanata. See W. B. Carpenter, 

 ' Challenger Reports,' vii. ' On the genus Orbitolites! The spiral origin of the test is indicated 

 plainly but slightly in the perforate Cycloclypeus, and apparently not at all in Orbitoides. The most 

 remarkable feature in Cycloclypeus is the addition of a fresh calcareous lamina to each surface of the 

 test when a new cycle of chamberlets is formed ; and in Orbitoides the presence of irregularly arranged 

 chambers on either aspect of the disc. 



