144 FAMILY LITUOLIDA. 



conformation of that of Truncaiulina (Plate XI, fig. 13). But we pass from these to a series 

 of forms in which the shell becomes more and more symmetrical, completing itself on both 

 sides, and at last detaching itself altogether, and assuming a regular nautiloid form (Plate VI, 

 figs. 39, 40). This very interesting type is met with at the present time on our own coasts, 

 and very abundantly in some northern seas ; it possesses so exactly the characteristic form of 

 Nonionina, that it has been unhesitatingly referred to that genus by Profs. Schultze 

 (xcviii) and Williamson (ex, p. 34) ; and yet I feel myself bound to agree with Messrs. 

 Parker and Rupert Jones as to the place they have assigned to it, since, after a very careful 

 examination, I have satisfied myself that the shell is altogether imperforate ; and that the 

 animal must therefore differ essentially from that of a Nonionina. The nautiloid spiral, more- 

 over, frequently gives place, in the fossil forms of this genus, to a rectilineal mode of growth, 

 so that the shell comes so strongly to resemble the " spiroline " form of Teneroplis as to have 

 been mistaken for it, the 8jpiroli7ia agglulinans of D'Orbigny (lxxiii, pi. vii, figs. 10, 11) 

 being nothing else than a Lifuola of which the chambers are undi\'ided. The septal orifice, 

 in all the foregoing varieties, is single, but varies in form from a simple, roundish passage, on 

 the one hand to a narrow fissure (Plate VI, fig. 40), on the other to an oblong, lobed, or 

 somewhat dendritine aperture. It varies also in position ; for whilst in the " nonionine " 

 varieties it is usually close to the inner margin of the chamber, it not unfrequently moves, 

 even in them, towards the middle of the septal plane, at the same time becoming more 

 crescentic (fig. 46), whilst in the straight portion of the " spiroline'' variety it is commonly 

 central. 



214. From these simple varieties of conformation, we now pass to a more important 

 modification of the fundamental type ; that, namely, which consists in the subdivision of the 

 chambers by secondary partitions, passing at right angles to the principal septa, as shown in 

 Plate VI, fig. 44 a, which represents a specimen of the Litiiola nautiloidea of Lamarck, par- 

 tially laid open by the wearing-away of its outer layer. Now this subdivision varies so much 

 in its kind and degree in different individuals — being most complete and regular in the largest 

 forms, whilst it is only present in a rudimentary condition in those which are less developed, 

 — that its existence cannot be fairly regarded as involving more than a varietal differentiation 

 of the forms in which it occurs. The extent to which it is carried may be generally deter- 

 mined by an examination of the septal plane ; which presents a multiplication of apertural 

 pores corresponding to the subdivision of the chambers. It is worthy of note that in this 

 labyrinthic variety even the secondary partitions, like the principal septa and the external 

 wall, are formed of arenaceous material. One other remarkable variety remains to be noticed, 

 in which the growth is rectilineal from the first (Plate VI, fig. 43), its chambers being sub- 

 divided, as in the preceding form. This is connected with the preceding by a series of inter- 

 mediate gradations, in which the " nautiloid " or spiral commencement of the shell bears a 

 smaller and smaller proportion to its straight or " nodosarian " extension, and at last comes 

 to be altogether wanting. The septal plane is so extremely convex as to be almost conical, 

 bearing at its summit the aperture, which is usually composite (fig. 45), or, if single, is a large 

 dendritic fissure. This peculiar form is common in many Tertiary deposits, and also occurs 

 in tropical seas at the present time. 



