MILIOLIDA. 21 



Characters.— ^\ie\\ ear-shaped, reniforrn, or orbicular, compressed, thickened at the 

 iirabihcns. Chambers arranged spirally, usually divided into chamberlets. Pseu- 

 dopodial orifices in one or more rows on the peripheral edge of the last chamber. 

 Surface of the chamber frequently marked by delicate parallel transverse riblets. 

 Diameter, jgth to |th inch. 



Orbiculina flourishes in warm seas, but seems to be very rare in the Mediterranean. 

 It is sparingly found in some of the European Tertiaries. Only one specimen, small and 

 reniform, has occurred to us from the Crag (Sutton?). 



2. Orbiculina compressa, lyOrbigny. Plate III, fig. 43. 



Orbiculina compuessa, D'Orb., 1840. Foram. Cuba, p. 66, pi. 8, figs. 4 — 7. 

 Orbitolites coscinodiscus, jS. Wood, 1843. Jlorris's Cat. Brit. Foss., p. 42; 1844, Mag. 

 N. Hist., vol. xiii, p. 21. 

 — (?) — Id. 1854. Morris's Cat. Brit. Foss., 2nd edit., p. .39. 



Characters. — Shell complanate, discoidal. Earlier chambers arranged spirally, as in 

 the type, later chambers cyclical. Chambers subdivided into chamberlets. 

 Diameter, ith inch. 



Although in localities where Orbiculines are plentiful, specimens of a large size are 

 often found retaining the spiral arrangement throughout their whole series of chambers, 

 we more frequently find that those which attain the finest proportions have assumed an 

 outspread discoidal form, in place of the ear-like or reniform .shape, owing to the 

 alteration in the plan of development before alluded to. When this change commences, 

 as is often the case after a very few chambers have been formed, a thickening of the 

 umbilicus is almost the only external character which will enable us to separate the speci- 

 mens from those of the closely allied genus Orbitolites, and even this feature may be 

 wanting. Microscopical examination of tlie central or umbilical portion of the disk 

 usually yields a ready means of determining the affinities of doubtful specimens in the 

 arrangement of the early chambers. Orbiculina has invariably a nucleus of spirally 

 arranged segments, however large and outspread the finely grown specimen may be ; 

 whilst Orbitolites, commencing growth with one or two large chambers, is built up entirely 

 of concentric bands, in even the smallest and most obscure examples. 



Specimens of 0. compressa were not rare in the Crag at Sutton some years ago, 

 when worked at by Mr. Wood. The figured specimen is of large size, but somewhat 

 worn and broken. 



Mr. Wood, in his " Catalogue of the Zoophji^es from the Crag," ' Mag. Nat. Hist.,' 

 1844, vol. xiii, p. 21, describes, under the name of Orbitolites coscinodiscus, some 

 specimens of this Foraminifer obtained at Ramsholt and Sutton. It is there stated that 



