106 FAMILY MILIOLIDA. 



155. External Characters. — The calcareous disks of Orbitolites (Plate IX, figs. 1, 7) vary 

 in diameter from about l-30th of an inch to ^-lOths of an inch ; whether large or small, 

 the}' are almost invariably circular or nearly so ; they are usually flat or slightly biconcave, 

 any difference in thickness being generally in favour of the marginal portion ; and if, as 

 sometimes happens, there is a slight central prominence (fig. 7, a), this consists only of that 

 first-formed portion of the shell which I shall describe as the " primitive disk" (^ 157), and 

 does not result (as in Orbiculina, ^ 135) from any overlaying of the original centre by subse- 

 quent growths of shell. Around this " primitive disk" we see an indeterminate number of con- 

 centric zones marked out by farrows of the surface ; and each of these zones is subdivided by 

 transverse furrows into a uniform series of areolae, which correspond with " chamberlets" 

 within. In the smaller specimens (fig. 1) these areolae are usually somewhat ovate in form, 

 their long diameter lying transversely to the radius of the disk ; in the larger specimens 

 (fig. 7), on the other hand, they have usually a narrow, rectangular shape, and their length lies 

 in the direction of the radius of the disk. This difference will presently be shown to be related 

 to an important diversity in internal structure. In those disks which have their surface 

 divided into ovate areolae (fig. 1), the margin presents a series of convexities with inter- 

 vening furrowed depressions ; whilst in the thicker disks (fig. 7) these are less regular and 

 less strongly marked. Unless the surface of the disk has been partially or entirely removed 

 by abrasion (as is often the case with dead specimens collected by dredging), it presents no 

 openings whatever, and the only orifices by which the interior of the shelly disk normally 

 communicates with the exterior are the pores (figs. 1, 7,f,f) that are seen upon its margin. Of 

 these pores, in the thinnest specimens (fig. 1), we find but a single one, or in some instances 

 two, lying in each of the marginal hollows ; in disks somewhat thicker we find each furrow 

 to contain three or four such pores ; whilst in the largest and thickest disks (fig. 7) the 

 number of pores occasionally rises to ten or even twelve. Thus, on the one hand, by the form 

 of the superficial areolae, on the other, by the singleness or multiplicity of the rows of 

 marginal pores, there is marked out a distinction between two types of Orbitolite structure 

 which have been accounted specifically or even generically different, but which, as they 

 frequently coexist in the same individual, have no title to be so regarded. It will be conve- 

 nient, however, to describe them separately, in the first instance, as the Simple and the 

 Complex types ; and to consider their mutual relationship subsequently. In both these types 

 the aspect of the shell much resembles that of Orhiculina, but has usually less of the opaque 

 whiteness by which it is characterised in that genus ; and sometimes, as in Orhiculina, the 

 surface is minutely punctated, giving a semblance of perforations, which, however, have no 

 real existence. In very thin specimens the calcareous laminae which form the two surfaces of 

 the disk are so translucent that, when such specimens are mounted in Canada balsam, the 

 chambers they enclose can be clearly made out. Moreover, even in larger specimens which 

 have been collected alive, and which consequently contain the animal body in a desiccated 

 state, the translucency of the shell allows the crimson hue of the sarcode to be seen through 

 it, so that the whole disk seems to be tinged by this, although the proper substance of the 

 shell, when examined by transmitted light under a sufficient magnifying power, is found to 

 present the brownish-yellow hue characteristic of the Milioline type. 



