ORBICULINA ADUNCA. 135 



by successive concentric annuU of chambers. D'Orbigny's fig. 4 (0. compressa), 

 pi. viii, ' Foram. Cuba,' illustrates an analogous discoidal specimen ; and his fig. 5 

 is one which retains evidence of the central spire, just as is also seen in Brady's 

 fig. 9, pi. xiv. Report ' Challenger.' Fig. 43 is equivalent to Carpenter's fig. 11, 

 pi. xxviii, ' Phil. Trans.,' vol. cxlvi, 1856. Fig. 44 is an approximately adult 

 individual of this variable form, which changes much as to details in closing up its 

 subdiscoidal shell by the meeting and coalescence of the newest marginal 

 chambers. This specimen stands close to such forms as are represented by 

 figs. 7 and 8 in pi. xiv, Report ' Challenger,' and described by Brady at p. 209; 

 and is equivalent to Carpenter's fig. 4, pi. xxviii, 'Philos. Trans.,' vol. cxlvi, 1856. 

 Fig. 44 is the specimen P. 3094 in the British Museum. PI. vii, fig. 4, is a broken 

 specimen from Sutton, which evidently had much more of the spire exposed than 

 the specimen illustrated by pi. iii, fig. 44, and in this respect it has a relationship 

 with Peneroplis (?) prisca, Reuss (' Denksch. k. Akad. Wien,' vol. xxiii, 1864, p. 9, 

 pi. i, fig. 7), from the Eocene of Styria. The shape of the chaaiberlets, which are 

 worn open in our fig. 44 and fig. 4, is like that of Orhitolites fenuissimus, Carpenter ; 

 but this is evidently a modification of the shorter chamberlets in fig. 43, and need 

 not be mistaken for the externally striate structure of Peneroplis} 



Occurrence. — Orbiculina aclunca is essentially a tropical form. It frequents 

 shallow waters, and the greatest depth at which it has been found is 450 fathoms. 

 As a fossil it is rare, and has been recorded with certainty only from the Miocene 

 of Vienna. In the Coralline Crag the true adunca has been found at Sutton, 

 zone e. The specimens of the complanate variety, PL III, fig. 43, were also 

 obtained from Sutton. 



^ Commenting on the absence of rigidity in adherence to a type-form, which has been noticed in 

 the genera and species of the Foraminifera by many observers, Dr. A. Goes remarks (in a letter), 

 " If we take, for instance, Peneroplis and Orhitolites we see a constant oscillation going on between 

 the two forms per medium of Orbiculina, — Orhitolites breeding its own form, also Orhiculina, and 

 perhaps also Peneroplis." In the progress (with atavism) to a higher type, the primal form becomes 

 " a larval stage to the latter until it becomes reduced to a single embryonal chamber, and the new 

 type is ready. The same with Miliolina, Qlohigerina, Uvigerina, Cristellaria, Frondicularia, 

 Textilaria, &c. This oscillation is best seen in Orhiculina, because there the primal type and the final 

 type are included in the same shell. We thus see that many of our ' genera' and ' species ' are not 

 founded on natural principles." 



