On the Carboniferous shark Tristychius arcuatus Agassiz from Scotland

SynopsisSkeletal remains of Tristychius arcuatus commonly occur in ironstone nodules from the Scottish Upper and Lower Oil Shale Groups. This material is clearly distinguishable from the small shark from Glencartholm described by Traquair, Woodward and Moy-Thomas under this name. Study of the latter shows that its finspines are distinctly different from the holotype of T. arcuatus and, therefore, it has been renamed.Tristychius arcuatus sensu stricto is a medium-sized hybodont shark with a short gape, a functionally heterocercal tail and narrow-based, tribasal pectoral fins. Its most unusual feature is a well developed opercular gill cover composed of long hyoid rays. Evidence suggests that this character was present in several Palaeozoic sharks, although it is absent in all Recent elasmobranchs. It is not clear whether it was primitively present in chondrichthyans or evolved separately in several lineages.Hybodonts and ctenacanths are recognised as separate, specialised shark radiations, neither of which can be directly ancestral to Recent sharks. Of the two, hybodonts appear to be more closely related to Recent forms, although the presence of typical hybodont finspines in Tristychius arcuatus indicates that they had diverged from ancestral euselachians before the beginning of the Carboniferous.

14.—The Silurian Rocks bordering Kirkcudbright Bay, South Scotland

SynopsisThe Riccarton and Raeberry Castle beds in Kirkcudbrightshire have hitherto been regarded as Wenlock in age, and the Hawick Rocks as Llandovery. Detailed mapping and revision of the graptolite faunas shows that the Raeberry Castle beds are Llandovery and the Riccarton beds Wenlock in age. On structural evidence the Hawick Rocks south of Kirkcudbright are considered to be younger than the Riccarton beds. The Riccarton beds in the area are formally defined as the Ross Formation and the Raeberry Castle beds as the Raeberry Castle Formation. Directional data suggest that the sediment forming the Raeberry Castle Formation was derived from the west and the sediment forming the Ross Formation and Hawick Rocks was derived from the north-east.

12.—Further Observations on some Lower Carboniferous Seeds and Cupules

SynopsisTwenty-eight specimens of the seed genusDolichospermahave been examined. Fourteen are assigned toD. sexangulatumLong; these possess six or seven integumental lobes which extend apically as free tentacles equal in length to the seed body except when damaged or broken off. Fourteen other specimens are namedD. pentagonumsp. nov. These are smaller and usually have five integumental lobes (less commonly six or seven). Long apical tentacles have not been seen in this species.Evidence is adduced thatAnasperma burnenseLong is the mature prothallial seed developed from the ovuleEurystoma trigonaLong.Eurystoma angulareLong sometimes possesses five integumental lobes and the free apical lobes vary in length.Compressed megaspores macerated from cupules ofStamnostoma huttonenseLong sometimes have three abortive spores in a compact group at the apex.Ovules ofEccroustosperma langtonenseLong were borne in small bunches (probably pendent) on short pedicels; one specimen of this seed shows three well-developed separate abortive spores around the apex of the functional megaspore.The cupule ofCalathospermum fimbriatumBarnard shows a structure suggesting that it may have evolved from a pair of cupules. Twelve to fourteen ovules were sometimes formed in the entire cupule though most of these were probably abortive.Some possible seed embryos are described and discussed, and an outline classification of seeds in the order Lagenostomales is proposed.

13.—Zoning of Ore Deposits: Types and Causes

SynopsisThe zoning found in ore deposits is dominantly of two distinctive types: that in syngenetic sedimentary deposits, like the Kupferschiefer, and that in common hydrothermal deposits of either epigenetic or syngenetic characteristics. Here, epigenetic hydrothermal deposits include those commonly classified as vein, porphyry copper, contact metasomatic, and Mississippi Valley deposits; the syngenetic hydrothermal deposits are conformable, massive ores such as at Rammelsberg. The two zoning sequences, beginning nearest the source of the metals, are: in hydrothermal deposits, Fe—Ni—Sn—Cu—Zn—Pb—Ag—Au—Sb—Hg, and in sedimentary deposits, Cu—Ag—Pb—Zn.A zoning sequence represents the natural order of increasing solubilities of the metallic sulphides and other minerals in ore-forming solutions. Comparison of zoning sequences with relative solubilities in proposed ore solutions provides a rigorous test of the efficacy of such solutions. When corrected for relative metal concentrations (mass-action effect), then both theoretically predicted, and experimental relative solubilities of sulphide complexes match precisely the order of hydrothermal zoning. The order in sedimentary zoning is identical to the mass-action-corrected calculation of the sequence in which sulphides must precipitate from solutions containing metallic ions or weak chloride or hydroxyl complexes. The consanguinity of these correlations imply (1) that hydrothermal zoning is the product of deposition from sulphide complexes, and (2) that chloride complexes may be the metal-transporting agent in sedimentary deposits.