10 DE. CAEPENTEE'S EESEAECHES ON THE FOEAMINIFEEA. 



by the later, varies as much as the degree of turgidity ; for whilst in some of the most 

 compressed forms, as in Peneroplis, each whorl does little more than apply itself 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 cavity 



(Fig. III. B, D). 



135. Thirdly, we observe in Dendritina precisely the same tendency to rectilineal 

 extension in the later period of growth, as in Peneroplis ; for although a distinct generic 

 term Spirolina 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. 11, bears just the 

 same relation to the typical Dendritina, that those shown in figs. 5, 7 bear to the typical 

 Peneroplis. The transition from Dendritina to Spirolina is well seen in figs. 12, 13; 

 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 Dendritina ; 

 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 into Spiro- 

 lincB. 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 spiral mode 

 of growth, has all the appearance of an ordinary Dendritina. 



136. Still it may be said that, notwithstanding all these points of resemblance, the 

 difference between Peneroplis and Dendritina is clearly marked out by the difference in 

 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 freely admit 

 that this would be the case, if the difference were constantly to present itself 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 Peneroplis, 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 irregular shape. On 

 the other hand, among the unquestioned Dendritince, 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. II. B, D, where the proportions of the aperture 

 are altogether reversed, its breadth being much greater than its length, and its central 

 part being enlarged at the expense of its ramifications ; while through such an aperture 

 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 



