Ammonites from a transported mass of Jurassic clay at Plaidy, Aberdeenshire, with the description of a new species

Abstract

The object of this communication is to place on record the occurrence of several ammonites hitherto not recognised in Scottish Jurassic rocks, and to describe an interesting new species. The shells were collected many years ago by the late A. Macconochie from the large erratic of Jurassic clay exposed in a brickyard north of Plaidy station, Aberdeenshire.2 The clay, which is dark blue in colour, contains blocks of grey or brownish-grey limestone, and is covered by a reddish sandy bouldery drift. It is clear, therefore, that it is a transported mass enveloped in glacial drift, as Jamieson maintained.3 To judge by the matrix, the ammonites appear to have been obtained from a single block of limestone, and were associated with other shells, chiefly those of lamellibranchs. The fossils are beautifully preserved, the specimens still retaining their original iridescent shell substance. The fauna, which includes the following species, Amoeboceras ( Prionodoceras ) aff. alternoides (Nikitin), A. ( Prionodoceras ) excentricum (S. S. Buckman), A. ( Prionodoceras ) aff. superstes (Phillips), A. ( Prionodoceras ) sp. nov. (see below) and Euaspidoceras aff. rotari (Oppel) indicates the Zone of Amoeboceras serratum of the Upper Corallian; the clays and limestones of the erratic at Plaidy, therefore, are the equivalent of the Ampthill Clay of England. It is worthy of mention that Amoeboceras ( Prionodoceras ) serratum is a shell most commonly found derived in the Boulder Clay of the Midlands of England, and this fact seems to suggest that the Serratum Beds possess a lithological character which enables them to be detached in blocks