EEPORT ON THE GENUS ORBITOLITES. 25 



the chambers around the " peneropline " umbilicus, so that they completely enclose it 

 annularly. But the chambers, instead of being partitioned into chamberlets, show only 

 the indications of subdivision which are marked on the " internal casts " of the sarcodic 

 body^ as slight constrictions ; — the type thus presenting as complete a link between the 

 undivided " peneropline " and the labyrinthic " orbitoline " systems of chambering as it 

 does between their respective geometrical plans of growth. It occurs in the " sables de 

 Fontainbleau, " near Rennes, which form part of the " Oligocene " Tertiaries. 



3. Orhitolites duplex, Carpenter (PI. III. figs. 8-14 ; PL IV. figs. 6-10; PI. V. 

 figs. 1-10). 



Amphisorus hempricJui, Elireuberg, Familien und Gattungen der Polythalamien. in Abliaudl. 



der konig. Akad. der Wissenscliaften zu Berlin, 1839. 

 Orhitolites, duplex type, Carpenter, Pldl. Trans., 1856, pp. 220, 224, and Introd. to Study of 



Foraminifera, 1862, p. 118. 

 Orhitolites macropora (t), Lamarck, Animaux sans Vert^bres, ed. 2, torn. ii. p. 196 ; figured in 



Goldfuss's Petrefacta, pi. xii. fig. 8. 



In my former Memoir (Phil. Trans., 1856, §§ 4, 59, 68) I indicated the existence of a 

 well-marked type of Orl^itoline structure, which differs from the ordinary "simple" type 

 in having a double series of marginal pores, and from the " complex " in the limitation 

 of the pores to two rows. My knowledge of this duplex type was at that time chiefly derived 

 from the small and worn specimens of it which I had picked out of some shell-sand 

 brought from the Red Sea ; and these I could pretty certainly identify with the forms 

 on which Prof Ehrenberg had founded his genus Amphisorus, and which he had ranked 

 with his Sorites {Orhitolites marginalis) among Bryozoa. But the large number of 

 unworn specimens of this type — many of them alive when captured — that are contained 

 in the collection made in the 18 fathoms' dredging of the Challenger on the Fiji reef, 

 enables me now to furnish a more accurate and complete account of it than it was 

 formerly in my power to give.^ As this type is sufficiently and constantly differentiated 

 by the character I have just specified, I designate it as Orhitolites duplex. 



The disks of this species (PI. III. fig. 8) have usually a very regular circular form, and a 

 nearly plane surface ; their thickness being almost uniform, with the exception that the inner 

 or central portion of any disk is usually rather thinner than its outer or peripheral portion. 

 The greatest diameter I have met with in the disks of this species is 0*32 inch, and the 

 greatest thickness 0"012 inch, the proportion of the two dimensions being thus that of a 



1 1 am not fully satisfied that I am correct in my interpretation of the structure of this fossil ; the shell of -n-hich seems 

 to me to have undergone the same kind of softening that is common in that of deep-water Miliolines, whilst the cavitary 

 system appears to have been occupied by a calcareous deposit of much firmer consistence. 



- In my former description of it, I fell into the error of supposing that the doubling of tha series of pores indicates 

 the existence, not only of two tiers of chamberlets, but of two annular canals. There is, as I shall presently show, only 

 a single annular canal, and, strictly speaking, but a single series of chamberlets, although there is frequently a want of 

 continuity between the upper and under portions of each cylindrical cavity. 



(ZOOL. CHALL. EXP. — PART XXI. 1883.) X 4 



