MILIOLIDA. 15 



Suhgenus — Spirolocouna, lyOrhigny. 



General characters. — Shell consisting of numerous segments arranged spirally on one 

 plane. Segments scarcely embracing, so that the whole number are visible on both 

 lateral faces. 



1. Spiroloculina plaxulata, Lamarck. Plate III, figs. 37, 38. 



MiLioLiTES TLANUi.ATA, Lam., 1805. Ann. Mus., vol. v, p. 3.52, No. 4. 



MiLiOLA — Hefr., 182-4. Diet. Nat. Sc, xxi, p. 68. 



bPiKOLOccLiKA DEPUESSA, JfOrh., 1826. Modele No. 02. Ann. Sc. Nat., vol. vii, p. 



298, No. 1 ; Soldani, Test. Zoopli., vol. iii, p. 229, pi. 155, 



figs. Tc k. 



— Badenensts, Id., 1846. For. Foss. Vien.,p. 2/0, pi. 16, figs. 13— 15. 



— DiLATATA, Id., 1846. lb., p. 271, pi. 16, figs. 16— 18. 



— EXCAVATA, Id., 1846. lb., figs. 19— 21. 



— CONCENTKICA, -S. Wood, 1843. Morris's Cat. Brit. Foss., p. 64. 

 ■ — DEPUESSA, Junes, 1S.")4. lb., 2ncl edit., p. 43. 



— — var. ROTUNDATA, Williamson, 1858. Rec. For. Gt. Brit., p. 82, 



pl.7, fig. 178. 



— PLANULATA, ParJcer &\\A Jones, 1860. Ann. N. Hist, 3rd ser.,Tol. v, p. 466 ; 



P. J. and Brady, 1865, ib., vol. xvi, p. 33. 



Characters. — Shell elliptical or oblong, complanate ; cliambers all visible ; margins 

 more or less rounded. Length, gth inch. 



This is the central, sub-typical form of the Spiroloculinm. Amongst the fine bold 

 specimens belonging to this group there is less variation from the normal condition than 

 in any other of the Milioline sub-genera. The chief deviations which we find are those 

 arising from feeble growth, giving rise to an elongated starved condition of the shell ; or, 

 as a result of rapid development from very small central chamber, an extremely bi-concave 

 form in the adult. There are also occasional irregularities in the contour of the shell, 

 from the much curved or siginoidal growth of the chambers, and from the hollowing of 

 their lateral faces ; but the absence of surface-marking from the entire group lessens the 

 number of varieties requiring trivial names. 



Spiroloculina planulata is common in the Sutton Crag, and the specimens obtained 

 from that source are of good size and of coarse growth. From the Polyzoan Crag we 

 have seen only a few small examples. Geologically, Spiroloculina appear amongst the 



