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Nature volume 114, pages 145148 (26 July 1924) | Download Citation

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Abstract

LONDON. Geological Society, June 25.—Dr. J. W. Evans, president, in the chair.—Miss M. E. Tomlinson: The river-deposits of the lower valley of the Warwickshire Avon; with an appendix by A. S. Kennard and W. J. Woodward. Fluviatile sands and gravels occur as river-terraces between Stratford and Tewkesbury. The surfaces of these terraces lie on curves parallel to the present thalweg of the Avon. Five such terraces have been identified. No. 5 Terrace, the highest above river-level, has yielded no contemporaneous fossils. No. 4 and No. 3 both contain a warm fauna, with Hippopotamus and Belgrandia marginata occurring solely at No. 3 level, and Corbicula fluminalis at No. 4 level. No. 2 contains a cold fauna, with numerous remains of mammoth and Rhinoceros tichorhinus. No. 1 probably underlies the alluvium, and may be connected with the infilling of the buried channel which has been identified at Fladbury and Tewkesbury. The sole evidence of human industry found is a fresh flint of Mousterian type from No. 2 Terrace.—F. Raw: The development of Leptoplastus salteri Callaway, and of other trilobites (Olenidæ, Ptycho-paridæ, Conocoryphidæ, Paradoxidae, Phacopidae, and Mesonacidae). Leptoplastus salteri occurs in the Upper Tremadoc horizon of Shineton Brook (Salop). All stages of development are noted, except the protaspis. The development is divided into successive stages. The earliest stage has a long, simply segmented glabella; long pleural spines occur throughout the body, including three pairs on the head and four on the pygidium; while axial spines occur from the occipital segment throughout. In the last stage the glabella is short, conical, and smooth; of pleural spines, only the genal remain on the head, those in the thorax are reduced, and none remain on the pygidium, where axial spines are also- wanting. With certain reservations, these four stages are also claimed to represent successive stages in the phylogeny. This ontogeny sheds much light on that of other trilobites. In the ontogenies of several families, it is the primitively posterior head-spines that at first are dominant to be superseded later by the middle pair. In Mesonacidæ these again are succeeded by the anterior pair, which, in their lateral revolution, carried before them the anterior branches of the facial sutures, so that these cut the posterior border; they also stretched out the antero-marginal suture to the genal angle. These changes led to the supersession of the dorsal facial suture by the marginal suture, with consequent ankylosis along the former. The family is hereby claimed to have a head-structure not hitherto recognised. It illustrates a general principle—that those only of the cephalic sutures are retained as were necessary for ecdysis. The combined study of trilobite ontogeny and morphology strongly suggests, as Beecher claimed, that here the ontogeny extensively recapitulates the phylogeny; and therefore best indicates relationships. This indicates an early divergence in glabella-form, antedating divergences in cephalic spines and sutures. Members of a “natural “order may, therefore, be expected to agree much more in glabella-form than in the cephalic suture, which has latterlv been made the basis of classification.

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https://doi.org/10.1038/114145b0

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