LITUOLIDA. 25 



Family— LIT UOLID A, Carpenter. 



Genus — Trochammina, Parker and Jones. 



Webbina, D'Orb. (in part). 



RoTALiNA, Williamson (in part). 



Trochammina, Parher and Jones, Reuss, Carpenter, and Brady. 



Ammodiscus, Reuss (?). 



General characters. — Shell free or attached, very variable in form, consisting of one 

 or many chambers. Texture arenaceous, the sandy constituents being held together by 

 an ochreous cement, and not projecting above the surface, which is smooth. Poly- 

 thalaraous varieties have no proper septa ; but the division into chambers is effected by 

 constriction or infolding of the primary shell-wall. 



The genus Trochammina differs from Lituola and the other arenaceous genera in the 

 fact that, although its walls are chiefly built up of sand-grains, the particles are so 

 incorporated in the calcareous cement that the surface of the shell is usually quite smooth. 

 The solitary specimen, on the strength of which we accept Trochammina {IFebbind) 

 irregularis as a Crag species, is perhaps the most obscure form of the genus, and one 

 which may be readily overlooked. It consists of a minute, subconical, tent-like, circular 

 disc, growing parasitically on a flat bit of shell, and presenting no character to arrest the 

 attention. Indeed, it is only by the knowledge gained in the examination of a large 

 number of specimens that we are enabled to recognise its affinities, or even to satisfy our- 

 selves of its belonging to the Foraminifera. 



The simplest forms of Trochammina belong to a species {T. \_Wehbina\ irregularis, 

 D'Orb.) of which we have four varieties ; and, since it is useful to have a " subgeneric" 

 name distinguishing them from Trochammina proper (as is the case with so many other 

 Foraminiferal groups), we have proposed (' Phil. Trans.,' 1865, p. 435) to retain 

 D'Orbigny's term " Webbina," appHed by him to one of them, although first used for a 

 few-chambered, uniserial, curved form of Nubecularia rugosa ('Foram. Canaries,' p. 126, 

 pi. 1, figs. 16—18 ; and ' For. Foss. Vien.,' p. 74, pi. 21, figs. 11, 12). 



1. Webbina irregularis, D'Orb., is adherent, moniliform, with more or less oval 

 chambers, and varies in the relative length of its stoloniferous connecting tubes, in the 

 number of its chambers, and in the straightness or curvature of their fine of growth. 

 Sometimes the stolons bifurcate, giving rise to a branching arrangement of a few cham- 

 bers, common in strata of Cretaceous age, the Oxford Clay, &c. (' Quart. Journ. Geol. 

 Soc.,' 1860, vol. xvi, p. 304; 'Carpenter's Introd. Foram.,' 1862, p. 141, pi. 11, 

 figs. 8, 9). 



2. Webbina irregularis alternans, P. & J., is adherent, and has the stolons issuing 



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