70 FAMILY MILIOLIDA. 



thus it happens that they mould themselves to the shape of the bodies to which they are adherent, 

 and that the plan of their own conformation varies accordingly. It is when growing on flat sur- 

 faces, such as those afforded by shells or sea-weeds, that Nubecularia best exhibit their characters 

 (Plate V, figs. 6, 13) ; when, on the other hand, they extend themselves over the projections of 

 fohated shells, or ensheath the stems and branches of Zoophytes, Corallines, &c., they often lose 

 all external traces of definiteness of plan (figs. 8, 9), and even their internal structure would 

 be unintelligible if the simpler forms did not supply the means of interpreting it. It is difiicult 

 to say what ought to be accounted the typical plan of conformation in this genus ; but as it 

 usually (though not always) commences in a sjjiral, which subsequently gives place to other 

 modes of growth, this may be conveniently adopted as the starting-point. The spiral shell 

 of Nubecularia differs from that of Cornuspira in several very important particulars. In the 

 first place, it is frequently deficient altogether on the attached side (figs. 1 — 3), so that the 

 cavity of the tube is there bounded only by the surface to which its edges are adherent, and 

 the form of its section is not circular, but semicircular. Sometimes, however, a layer of shell 

 is deposited on that surface, so as to close-in the tube (figs. 4, 1 1, 14) ; but this layer is so thin 

 as to require the support afforded to it by the surface to which it is adherent. On the other 

 hand, the -shelly substance of which the unattached side of the tube is composed is deposited 

 in unusual abundance, so as not only to form a very thick wall on the exposed surface, but to 

 fill up the grooves which would otherwise be left between the successive coils of the spire. 

 Thus it often comes to pass that the distinction between the coils is altogether obliterated, 

 and only from the general outline of the entire shell could it be supposed that a spire is 

 concealed beneath. This deficiency of shell on the attached side, with such an exuberant 

 thickness on its exposed side as masks its characteristic form, is observable not merely in the 

 spiral NubecularicB, but also in those curved, straight, zigzag, ramifying, and acervuline 

 varieties through which this generic type will be shown to range (fig. 9). 



89. Internal Structure. — In those forms oi Nubecularia which spread themselves over plane 

 surfaces, the internal structure is readily disclosed by detaching the shell from its adhesion, and 

 looking at it from the under or attached side ; for its cavity is then either laid open altogether 

 (fig. 1), or becomes visible through the thin pellicle of shell which covers it (fig. 4). In the 

 most regular spiral forms the spire commences in a spheroidal chamber, from which it is 

 separated by a slight constriction, and it then rapidly opens itself out, especially in the second 

 or third whorl. Its cavity is partially divided at irregular intervals by imperfect septa, formed 

 by inflections of the walls of the tube, which curve inwards with a convexity directed forwards, 

 but stop far short of meeting at the axis of the tube, so that a wide aperture of communication 

 is left between the adjacent chambers. When the spire enlarges, a dilatation is seen beyond 

 every one of these constrictions, the narrowed aperture of one chamber being received, as it 

 were, into the dilated base of that which succeeds it (fig. 5). This is for the most part better 

 seen, however, in the ^//-ff^^/// growth (fig. 10), to which the spiral convolution very commonly 

 gives place after having made two or three turns ; the axis of growth alone being changed, and 

 the structure of the tube and its partitions remaining the same. This straight mode of growth 

 may prevail from the commencement, individuals sometimes presenting themselves in which 

 the spire is altogether wanting. Sometimes the new chamber is formed at the side instead 

 of in the axial hue of the preceding, so that the direction of growth is suddenly altered. 



