GENUS PENEROPLIS. 91 



individuals in which the characters of the two types are so blended as to make it difficult, if 

 not impossible, to say to which they should be referred. Thus in the specimen (fig. 10) 

 already referred to as presenting a linear series of pores, the marked elongation of which 

 shows each to be formed by the coalescence or imperfect separation of two, there is an addi- 

 tional pair precisely in the situation of the alar prolongations in fig. 14, c ; and it is obvious 

 that the simple form of dendritic aperture there represented would result from the coalescence 

 of these pores. along the middle of the septal plane. Another example, also from a young 

 specimen, is shown in fig. 6 ; and it is readily intelligible how in such a case the subsequent 

 compression of the spire and the narrowing of the septal plane may cause the pores to 

 present the simple linear arrangement characteristic of PencropUs ; whilst if, with a 

 continuance of the early mode of growth, the separate pores of later septa should coalesce 

 into one aperture, we should have a Bendritina. In another young specimen, shown in 

 fig. 11, such a fusion of several pores into a single dendritic aperture has actually taken 

 place («), whilst another broad aperture is seen just below this. Again, in fig. 7 we see the 

 broad septal plane irregularly perforated by numerous pores, some small and rounded, others 

 large and irregular, each of the latter being obviously formed by the coalescence or imperfect 

 separation of two or three ; the turgid spire of Bendritina and the separate pores of Pene- 

 roplis here coexisting with each other, so as to make it difficult to say to which type the 

 specimen should be referred. Again, the transition is easy from the specimen represented 

 in fig. 10 to that shown in fig. 9, and thence to the one whose septal plane is shown in 

 fig. 15 ; here the coalescence has proceeded so far as to produce a number of separate 

 branching apertures, and nothing is wanting but the removal of the line of shell which 

 passes down the middle of the septal plane to unite these into the mo§t characteristic form of 

 the single ramifying orifice. This individual, like the one represented in fig 13, was already 

 beginning to assume the Spirolinr form, the rounded shape of the mouth showing that the 

 spire has detached itself completely from the previously formed convolutions ; in figs. 4, «, 

 and 5 are shown the septal planes of more advanced examples of the same type, which 

 present such a combination of the large dendritic apeilure of Bendritina with the isolated 

 pores of Peneroplis as to complete (in my opinion) the proof that no valid distinction can be 

 drawn between these two types, either from the number, the isolation, the position, or the 

 shape of the apertures in the septa. Hence it follows that not merely must the genera 

 Bendritina and Spirolina be relinquished, but that both these forms must be regarded as mere 

 varieties of Peneroplis platiafus. 



131. Affinities. — A sort of sketching out of Peneroplis may be traced, as we have seen, 

 in some forms of Nubectdaria (% 90) ; and the type becomes still more defined in Ferte- 

 hralina, some of whose forms can only be distinguished from Peneroplis by their undivided 

 aperture. If the elongated sht of the compressed Vertehralince were to be subdivided into 

 a linear series of pores as in the typical Peneroplis, or the circular aperture of the cylindri- 

 form variety were to be modified in like manner so as to resemble that of Spirolina, the tran- 

 sition would be complete. Again, we have seen that an approximation to the Peneropliform 

 type is presented in the cribriform subdivision of the aperture of a true Miliohi (f 110), 

 whilst a still closer relationship to it is exhibited in the Hanerine modification of the Milioline 

 t)rpe (^ 112). Looking to the inconstancy of the aperture in the Peneroplis type itself, we 



