OCR Text |
Show 46 MR. F. CHAPMAN ON SOME [Jan. 15, section, the umbonal centre being more prominent on one side than the other. This fact points to the tendency of this species to increase in an oblique or turbinoid spiral, such as is shown in all undoubted Amphistegince and not on the Nummuline plan. I venture to suggest that the peripheral figure of this form, as originally given by Fichtel and Moll \ is too symmetrically drawn, and it is easy to conceive how such a slight degree of asymmetry would be overlooked without the accompaniment of carefully prepared sections of the test. Another feature, moreover, brought out in the transverse sections of the test, and which helps to strengthen the evidence in favour of this form belonging to the genus Amphistegina, is the existence of the characteristic double cone-shaped non-tubulate portions of the test which form its central axis in transverse section (see Plate I. fig. 9). Whilst examining the median sections of A. radiata, the presence of true interseptal canals with many branchlets was detected (see fig. 10). In his ' Introduction to the Study of the Foraminifera,' Dr. Carpenter describes the various characters which distinguish forms of the genus Amphistegina, and of which a summary and comparison with the Rotaline type is given at p. 246. Here it is remarked that the " singleness of the septal lamellae is a most important additional link of affinity " to the group of the Rotalines. This statement, which may have been made through the examination of non-typical specimens, caused m e some doubt as to the validity of the claim of A. radiata to the Ampbi-stegine group. Upon preparing sections of typical specimens of Amphistegina hauerina from the Vienna Basin, which I possess through the kindness of Professor T. Rupert Jones, I found the same well-developed canal-system existing in the fossil forms (see fig. 11), of the true position of which as Amphistegince there can be no question, as were seen in the recent specimens of A. radiata. Therefore that apparently serious objection was satisfactorily removed, and, at the same time, additional facts were obtained, which show that, as far as the shell-structure is concerned, Amphistegina is as highly advanced in differential characters as is the shell of Nummulina. The only difference therefore that appears to exist between ordinary Amphistegince of the A. lessonii type (including A. hauerina) and the recent A. radiata is the remarkable modification of the segments in the former type of the outer layer on the inferior side of the test giving rise to the " astral lobes." The transverse sections of the Amphistegince generally, if taken accurately through the middle of the shell, exhibit" the large spherical primordial chamber with the succeeding more or less ovoid one. I especially mention this fact since several examples of the young tests of A. radiata have occurred in the peripheral whorls of adult specimens of that species, and are seen in both median and 1 Op. cit. pi. viii. fig. d. |