V.—Notes on Phacops Weaveri, Salter

Abstract

A re-examination of the type-specimens of Phacops Weaveri, Salter, in the Museum of Practical Geology, Jermyn Street, was recently rendered necessary in connection with my palæontological work on the Tortworth Silurian area, the results of which have been published in the November number of the Quarterly Journal of the Geological Society, 1908 (vol. lxiv, pp. 512–545), in a joint paper by Professor S. H. Reynolds and myself. A question had arisen in the course of our work as to the occurrence of this species above the Llandovery Beds, for Salter had stated that some of his figured specimens of pygidia (Mon. Brit. Trilob., pl. iii, figs. 2, 3) came from doubtful Ludlow rocks at Horse-shoe Farm, Tortworth. The species had, however, been founded by him previously on specimens from socalled ‘Caradoc’ [= Llandovery] rocks from Long's Quarry, Tortworth, and an outline figure of the pygidium, apparently a restoration based on the three specimens in Jermyn Street (Nos. 19220, 19221, 19222), had then been given. Subsequently a head-shield from the same quarry was figured in his monograph (pl. iii, fig. 1). It is clear that Salter based the species mainly on the pygidial characters, for the head-shield is dismissed with a few inadequate remarks, its resemblance to Ph. caudatus being considered to be very close. The original description given by Salter in 1849 runs as follows:—“P. laevis, capite quam in P. caudato, nisi lobis glabellae tumidioribus;—caudâ triangulari, ferè aequilatera, convexa, apice acuto haud mucronato, axi 13–16 annulate, costis lateralibus 10–12, simplicibus, vix curvatis, ad marginem aequalem angustum abruptè terminatis.” In the diagrammatic figure of the pygidium which accompanied this description only 12 or 13 rings are indicated on the axis and 10 or 11 ribs on the lateral lobes, which might incline us to think that an immature individual was chosen for the figure, since Salter in his remarks on the species says (op. cit.) that “young specimens have not the full number of ribs”, but no mention that this is the case is found in the explanation of the plate. In Salter's subsequent work (Mon. Brit. Trilob., p. 58) the tail is described as “broad-triangular, wider than long; the sides a little convex; the apex short-mucronate; the axis narrow, conical, ribbed by about sixteen rings; the sides very convex with nine to ten arched simple ribs scarcely at all interlined; the margin [= border] narrow, smooth”. The italics in this quotation are mine, as the number of ribs is fewer than that given in his earlier definition of the species. It may here be remarked that Salter in his monograph reproduces verbatim the definition of the species published in the decades, with the addition of the word trigono with reference to the head before the word nisi, and with the insertion of the epithet multicostata after the word œquilatera in the description of the pygidium.