GENUS PENEROPLIS. , 85 



is in great degree overlapped by tlic next ; and it very commonly happens that the last half 

 whorl spreads itself out to such a degree as entirely to invest the preceding, the extension 

 even reaching so far as not only to conceal the original umbilicus, but even to encroach upon 

 the opposite half of the spire (fig. 10). When such is the case, however, the extension 

 is usually limited to one of the lateral surfaces of the shell, which thus becomes unsym- 

 metrical. Although this extension is sometimes confined to the inner margin of the spire, 

 as is seen in fig. 16, yet it often occurs along the outer margin also, being usually limited, 

 however, to the last four or five chambers. The septal plane extends itself in some 

 instances around as much as three-fourths of the whole margin of the shell ; thus showing, 

 even in this type, an obvious tendency towards that cyclical mode of growth which we meet 

 with in Orbicidina, but which is essentiall}^ characteristic of OrbitolUes. The surface of the 

 shell is highly polished, and has a peculiarly opaque-white, porccllanous aspect. It is very 

 strongly marked by depressed bands, which indicate the places of the septa between the 

 chambers ; and between these septal bands the walls of the chambers rise in flattened arches. 

 In a direction transverse to the septal bands we almost uniformly observe a strongly marked 

 striation, the stria; running parallel at tolerably regular intervals, which average about 

 l-1400th of an inch, from one septal band to another; this striation, which imparts a very 

 characteristic physiognomy to these minute shells, seems due to a sort of plication or ridge 

 and furrow arrangement of the slielly wall (Plate VII, fig. 20), which may not improbably 

 have the effect of imparting to it increased strength. The plication generally disappears at 

 the junction of the walls of the chambers with the septa, and consequently we do not usually 

 see it at the septal plane when the shell is viewed endways. Sometimes, however, it is con- 

 tinued on to the septal plane itself; and its character is then extremely well displayed, as in 

 fig. 15, which, however, represents not the typical form, but one of the varieties to be 

 hereafter noticed. On the prominences of the plicse, there are frequently to be seen rows of 

 extremely minute punctations (fig. 20) ; these, however, are not the apertures of passages 

 through the shell, as might not unnaturally be supposed, but are mere depressions of its surface, 

 as I have ascertained by the careful examination of veiy thin sections. It is remarkable that 

 the plication of the shell is sometimes wanting, though the punctations may still present 

 themselves in rows, as shown in fig. 2 ; whilst, in other cases, not only are the plicEC deficient, 

 but the punctations are distributed uniformly over the entire surface, as shown in fig. 3. 

 That these variations are not indicative of any specific difference, is at once proved by the 

 fact that the shells which exhibit them in one part present the ordinary character of surface 

 in another. Similar punctations occasionally present themselves on the septal plane of 

 aberrant forms of the PenerojAls type. 



123. The septal plane is perforated by numerous isolated pores, which, in those 

 extremely compressed specimens that constitute the most typical examples of this generic 

 form, are arranged in a single linear series (figs. 16, «, 18) ; the number of these pores 

 depends upon the length of the septal plane, and thus it usually increases with the age 

 of the individual, each chamber opening externally by a larger number of pores than did that 

 which preceded it. The typical form of these pores seems to be circular, though they are 

 apt to present various departures from that shape ; they usually lie in a sort of furrow 



