176 Messrs. W. K. Parker and T. R. Jones on the 



water this species is represented by the variety Rotalia Menardii, 

 D'Orb., which is flattish, limbatc, and granular, and has for com- 

 panions the contracted varieties/?. Michelmiana,~D'Orb.,R. crassa, 

 D'Orb., and R. nitida, Reuss, together with intermediate forms. 

 In shallower water, especially on muddy bottoms, we find large, 

 smooth, flush-chambered and more or less limbate forms, such 

 as R. Partschiana, D'Orb., and R. Schreibersii, D'Orb. ; but the 

 former of these is often found at great depths. 



In the Laminarian zone this species often takes on an irregu- 

 larity of growth, many of the latter chambers elongating them- 

 selves without septa. Thus (becoming almost as simple as 

 Spirillina vivipara, Ehrenberg) it becomes the Planorbulina ver- 

 miculatciy D'Orb., Ann. Sc. Nat. vol. vii. p. 280. 



The two varieties of this species represented by D'Orbigny in 

 his 'Modeles,' No. 12 (R. punctulata, D'Orb.) and No. 10 (R. 

 pulchella, D'Orb., which is the same as R. Caribbaa, D'Orb.), 

 must be placed very close to the typical R. repanda (which, in- 

 deed, is intermediate between these two). . 



In this species the shell is for the most part very finely 

 pored; but when the septal face is much flattened, or the 

 shell takes on a wild growth, a number of large holes give a 

 punched aspect to the septum ; and even, in the vermiculate 

 forms, the whole of the under surface of the shell is thus coarsely 

 pertused. 



Varieties of this species occur fossil as far back as the Upper 

 Trias, in nearly all the clays of the Oolites, in the Gault and 

 Chalk, and throughout the Tertiaries. It may be found in all 

 seas ; and we have it from the Tropics, brought up from 2700 

 fathoms (Capt. Pullen's soundings). 



26. Nautilus sinuatus. Page 65, pi. 10. figs. a-d. " Fossil : 

 San Quirico in the Siennese." 



This is a small-sized variety of the Rotalia repanda. It is 

 more margined and flattened; it is limbate on some of the 

 septal lines. The early part of the spire is hidden here by 

 granules. This ornament is seen in other varieties on the upper 

 and occasionally on the under surface. The different degrees of 

 ornament produced by exogenous shell-growth in the margins, 

 in limbation, and in granulation, scarcely permit us to find two 

 specimens alike, to say nothing of the variability as to the size 

 and thickness of the shell and the number of chambers. Still 

 the species has a habit of its own, with a peculiar setting-on of 

 the chambers, and style of ornament, which help us to see a 

 specific relationship between forms at first sight very different, 

 and at the same time to recognize limiting distinctions between 

 the most aberrant forms and their isomorphs belonging to other 

 species. 



