Cranidial Muscle Scars of ‘Illæus’ proles var. shelvensis Whittard

Abstract

FROM the core of the Walsall boring¹ which was put down in 1935, Dr. and Mrs. A. J. Butler collected a large number of Silurian fossils. Among those from the Purple Shales of the Upper Llandovery there are internal casts of two trilobite middle-shields referable respectively to Phacops (Portlockia) elegans Sars and Boeck, obtained at a depth of 1,050–1,055 feet, and to Illænus proles var. shelvensis Whittard, from 982–987 feet down. The illænid, as shown in the accompanying figure, though rather larger than the individuals upon which Prof. Whittard founded the variety², differs from these only in the remarkable preservation of the impressions of what seem to be muscular platforms. The pair of these which occur at the front of the glabella are expanded anteriorly, and are covered with small depressions like those occurring at points of muscular attachment in modern Crustacea. About 0·6 mm. behind these well-marked muscular platforms under the posterior part of the glabella we find another pair that are poorly defined with small raised areas sometimes isolated, irregularly arranged, and not so high as those in front.