GENUS PATELLINA. 231 



but being prolonged inwards beneath that margin (.Plate XIII, fig- 17), until the peripheral 

 chamberlets lose their distinctness by merging into the secondary growth which occupies 

 the umbilical cavity. The chamberlets are prolonged in a radial direction so as to have a tubular 

 form ; but not unfrequently we find that as we trace them from without inwards two of them 

 coalesce into one, as often happens in Cymbalopora. Owing to the conical arrangement of the suc- 

 cessive annuli, only a small part of each (except the last) is seen where it crops out upon the sur- 

 face, the remainder being concealed by those above and below it. The external walls of the 

 chamberlets are perforated with a few minute pores ; but those which separate the chamberlets 

 and annuli from each other are altogether imperforate, so far as I have been able to distinguish. 

 — The interior of the cone, in the British specimens of this type, is described by Prof Williamson 

 as partly occupied by an exogenous deposit of hyaline shell-substance, possessing no regular 

 arrangement ; and this seems evidently analogous to that which fills up the umbilical vestibule 

 in the most developed forms of Botalia, and which tends to form a secondary chamber-struc- 

 ture in the " asterigerine" BiscorhincB. This umbilical deposit seldom fills the cone so com- 

 pletely as to conceal the last annulus, which is consequently the one best seen from the under 

 side, and frequently the only one there visible (ex, fig. 89). In the larger Australian forms it lias 

 a distinctly loculated structure ; its chambers, however, are not arranged with any regularity, 

 but constitute a system of intercommunicating lacuna hollowed out in the midst of a mass of 

 solid shell-substance ; and in this system the chamberlets of the peripheral annuli appear to 

 terminate at their Central extremities, just as the chambers of Cymhalopora communicate with the 

 umbilical vestibule. I find it impossible, however, to speak with the confidence I would 

 desire upon this point ; for notwithstanding the beautifully hyaline texture of this shell, its 

 very small size as a whole, and the minute subdivision of its parts, render the examination of 

 its internal structure very diflicult. The still more minute forms described by Messrs. Parker 

 and Rupert Jones (lxxix) under the names OrhitoUna simplex and 0. semianuularis, are 

 obviously young or imperfectly developed examples of the type that has now been described. 



405. Patellina concava : External Characters and Internal Structure. — From the 

 Nummulitic limestone of the Pyrenees I have fossil specimens which resemble the preceding 

 in form and in superficial characters, but which are of comparatively gigantic dimensions, 

 attaining a diameter of 1-4 th of an inch; these appear to me to be identical in structure 

 (so far as their metamorphic condition allows me to judge) with the fossil which has been 

 recently described by Mr. Carter (xxiii a) under the name OrhitoUna lenticular is, being re- 

 garded by him as identical with the species so named by Blumenbach, which had been first 

 described and figured by Deluc (' Journal de Physique,' torn. Ivi, p. 344). I believe them also 

 to be identical with the OrhitoUna conica of JJ'Archiac (' Mem. Soc. Gdol. France,' torn, ii, 

 p. 178), and the 0. conoidea of Gras, (' Foss. de I'lsere,' p. 51, pi. i, figs. 4 — 6), from the 

 Cretaceous beds of France, as also with the 0. discoidea of Gras and the 0. plana and 0. 

 mamillata of D'Archiac. As all these appear to be varieties of the 0. concava of Lamarck, it 

 seems convenient to adopt this last (as Messrs. Parker and Rupert Jones have done, lxxix) 

 as the specific designation of the type, since it expresses a form intermediate between its 

 conical and its discoidal varieties.* The form of this fossil (Fig. XXXVII, a, b, c) seems to 



* I must freely confess, however, tliat I am not as 3'et prepared to say with certainty what of 



