SUB-FAMILY ROTALINiE. 



201 



circular ridge of shell, so that the whole surface is marked out into minute areolje (fig. I, a). 

 The segments progressively increase in size, and the earlier whorls are scarcely at all invested 

 by the later; so that the whole spire remains visible on one lateral surface, whilst on the other 

 side nothing is seen except the last convolution. Generally speaking, the former (as in a 

 Turbo or Trochus) is more or less conical or convex, the primordial chamber occupying its 

 highest point, whilst the latter or growing side is more or less flattened ; and as it is ordi- 

 narily by the latter that the shell is adherent to the sea-weeds, zoophytes, shells, or stones, to 

 which the animal commonly attaches itself during life, this may appropriately be called the 

 lower surface, whilst the former, which is free, is distinguished as the upper surface (Fig. XXXII, 

 A, b). We agree with Prof. Williamson (ex, p. xvi) in thinking it desirable to retain these terms 

 as morphologically correct, in cases in which the side whereon the whole spire is visible is the 

 flattest (Fig. XXXII, d, e), the spire having more the form of a Conns than of a Turbo or 

 Trochus; notwithstanding that, the shell being attached by the flatter side, the position of the 

 animal is (so to speak) inverted. — As a general rule in this series, the anterior portion of the 

 wall of each segment forms the posterior wall of the chamber that succeeds it, the septal layer 

 being single and not being differentiated in texture from the ordinary chamber wall, so that 

 there is neither intermediate skeleton nor canal-system : to this rule, however, there is a 

 marked exception in the genus Rotalia, which approaches the NummuUnida in the duplication 

 of its septal laminre and in the existence of intraseptal spaces ; and there are many other cases 

 in which the usual porous texture of the shell gives place, in the portion of the chamber wall 

 forming the septal plane, to a hyaline plate that displays none but large perforations. 



Fig. XXXII. 



\aiious forms of Rotalines .- — a, PulcinuUda bademis, Czjek; b, Discorbina dimidiafa, Jones and Parker; c, jMmalLia puncltt- 

 luta, D'Oibigny ; D, Truiicalidina loliidida. Walker and Jacob ; a, TruiicaliiUna i-efulrjeiis, Montagu ; r, I'latmlim 

 Anmhiensis, D'Orbigny ; G, ttiscorbina bi-conanu, Junes and Parker. 



346. The form of the spire, and that of the segments of which it is composed, 

 may difl'er very widely. As already stated, the tiirbinoid form, in wliicli the spire is 

 somewhat depressed and more or less spreading, and in which the individual segments ai'e 

 more or less ventricose, may be considered as the one most characteristic of the group 

 (Fig. XXXII, b). But, on the one hand, the spire may be as high, as regular, and as compact, 

 as that of a Trochus ; the shape of the segments being such that the outer surface of each is 



26 



