GENUS DISCORBINA. 203 



tion of Cymbalopora ; but it is in PlanorbuUna, among the typical Rotalines, that the cychcal 

 plan most completely prevails over the spiral, and replaces it at the earliest period. The 

 truly Rotaline character of this type is evidenced by the frequency of its " truncatuline" and 

 " anomaline" forms, these being merely PlcuiorhuluKS whose development has been arrested. 

 In this genus, moreover, the growth may become altogether " wild," the segments being 

 heaped up, one on another, in an irregular or " acervuline" manner; and tlius we are led to 

 the genus Tinoporus, in which the primordial spiral is recognisable with difficulty, and the 

 piling of segments is combined with a mode of horizontal extension that seems properly 

 cyclical ; whilst in the genus Poli/trcma all trace of the primordial spiral seems lost, and the 

 mode of growth seems almost indefinite. In these cyclical and acervuline forms of the i?o/rt//«e 

 type the aperture of communication undergoes a complete alteration in shape, being no longer 

 a crescentic fissure, but a circular passage, which is sometimes furnished with a pouting lip 

 that may be elongated into a tube. — A still more remarkable departure frona the ordinary 

 type will be found in the genus Patellina ; of which the primitive form seems to be a hollow 

 conical spire with only two segments in each turn, but each of these segments subdivided 

 into chamberlets ; this gives place, in the more developed examples, to a regular cyclical 

 increase, and the hollow of the cone (which corresponds to the umbilical vestibule of the 

 ordinary Rotalines) is filled up by a more or less regular growth of secondary chambers. 



350. The intimacy of the relationship that exists between the several genera which 

 normally constitute the Rotaline series, is evidenced by the fact, that among tlieir very 

 numerous varietal forms isomorphs are frequently met with ; these isomorphs often so 

 closely resembling each other, that it is only after a careful examination of the gradational 

 affinities of each that its proper position can be assigned. The largest number of such 

 varietal forms is presented by PlanorbuUna, which has yielded several scores of pseudo-species 

 to authors ; next to it comes Discorbina ; then Puloinulina, which has from forty to fifty such 

 forms ; then Botalia ; then Calcarina ; and lastly Cymbalopora, from whose varieties only four 

 species have been erected. — Of the more aberrant genera, Tinoporus, Polytremn, and Patellina, 

 the first and third will probably yield a large number of varietal forms when they shall have 

 been adequately studied ; the second appears essentially amorphous. Even these genera are not 

 without examples of the isomorphism just alluded to : thus the Tinoporus baculatus so 

 extremely resembles Calcarina Spent/leri as to be readily mistaken for it by such as are not 

 familiar with the special peculiarities of each ; and we shall see that in the early stage of 

 growth there is no essential dissimilarity between them. 



Genus XIIL— Discorbina (Plate XIII, figs. 2, 3 ; and Williamson, Figs. 104, 105, 109 — 1 13). 



351. External Characters and Infernal Structure. — The genus Discorbina may be regarded 

 as presenting the characteristic features of the Rotaline series in their simplest, as Rotalia 

 does in their most developed, condition. Its shell is typically a turbinoid spire, formed by a 

 succession of vesicular segments, every one of which bears a strong resemblance to a 



