10 
DE. CAEPENTBE’S EESEAECHES ON THE EOEAAIINIEEEA. 
by the later, varies as much as the degree of turgidity ; for whilst in some of the most 
compressed forms, as in PeneropUs, each whorl does Uttle more than apply itseK to the 
margin of the preceding (Fig. III. A, c), the more turgid the spire becomes, the more 
completely (generally speaking) does it embrace the preceding, the alar prolongations 
of the chambers thus coming to bear a large proportion to their principal canty 
(Fig. III. B, d). 1 X 4-:r 1 
135 Thirdly, we observe in Dendritina precisely the same tendency to rectiimeai 
extension in the later period of growth, as in PeneropUs ; for although a distinct pneiic 
term Smrolina has been given to the form presenting this modification, I think it must 
be apparent to every one that the example delineated in Plate II. fig. H, bears just the 
same relation to the typical that those shown in figs. 5, 7 bear to tbe tj^iicai 
Peneroplis. The transition from Bendritina to Spirolina is weU seen in figs. 1^, 1 ; 
in the first of which we see the last whorl apparently about to disengage itself from the 
earlier ones, whilst in the second that disengagement has been completed. I cannot 
conceive that any one could refuse to regard either of these specimens as a Bendntma ; 
and yet it is obvious that their continued increase upon the plan which has already 
manifested itself in the formation of the later chambers, would convert them mto Spiro- 
lince. Such continued increase has obviously taken place in the specimen represented in 
fig. 11, which, up to the time of the substitution of the rectilineal for the spu'al mode 
of growth, has all the appearance of an ordinary Bendritina. 
136. Still it may be said that, notwithstanding all these points of resemblance, the 
difference between Peneroplis and Bendritina is clearly marked out by the difference m 
the modes of communication between the chambers of the two types ; and that such a 
difference is sufficiently important to constitute a valid generic character. I ffeely admit 
that this would be the case, if the difference were constantly to present ffself between 
all the individuals of the same type, as it does between their characteristic examples 
(Plate I. figs. 1, 2); but the fact is far otherwise. We have seen that among those 
which would be unhesitatingly ranked under the designation Penerophs, there is not 
only a tendency to multiplication of the rows of separate pores, but also an occasional 
fusion of two or more pores, so as to form a single large pore of u'regular shape. On 
the other hand, among the unquestioned Bendritinm, we observe not merely that the 
form of the single large dendritic aperture is extremely variable, but that it is frequently 
so simple as to suggest the idea of having been formed by the coalescence of a linear 
series of pores. The most characteristic forms of the dendritic aperture that I have met 
with are shown in Plate II. figs. 12 a, 13 a; two examples of a remarkable departure 
from this have just been seen in Fig. IL b, d, where the proportions of the apertime 
are altogether reversed, its breadth being much greater than its length, and its centra 
part being enlarged at the expense of its ramifications ; while through such an apertui’e 
as that represented in Plate II. fig. 22, we are conducted back to the ordinary type, 
though its form is much simplified. On the other hand, in Fig. II. a, c, we have 
marked examples of a narrowing and elongation of the aperture, with such a reduction 
