No abstract

A modern palaeobiological approach to the taxonomy, making full allowance for a wide range of variation, has allowed the distinction of 22 Jurassic trigoniid species in Europe, grouped into 7 genera. Most species were con®ned to shallow marine habitats ranging up to a few tens of metres in depth but were adapted to low energy environments as well as environments of moderate to high energy. The presence of external ornament on the shell¯anks was probably an adaptation to facilitate burrowing, but in the absence of useful external ornament other adaptations to increase burrowing ef®ciency may have been realized by the development of an elongate shape. Because their occurrence in Jurassic strata is only intermittent, con®dent inferences on evolutionary patterns are limited to the two commonest genera, Trigonia and Myophorella. Phyletic size increase has been recognized in the latter but not the former genus. The calculated species longevities correspond fairly closely with those established for Jurassic bivalve species in general. The mode of speciation is dominantly punctuational, but only one likely example of species splitting is recognized.

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The Hangman Grits of the Quantocks consist of a series of sand-stones, now, in most instances, practically converted into quartzites by the great pressure to which they were subjected during the Armorican movements, and occasional shales, which sometimes show cleavage. There are also beds, never of any great thickness, which bear a strong resemblance to the Keuper Marls. Fossils, other than a few obscure plant-remains, are unknown, except in the beds which are usually regarded as forming the top of the series, where marine fossils occur as casts in what is evidently a decalcified calcareous sandstone.

No abstract