The Development of Leptoplastus Salteri (Callaway), and of other Trilobites (Olenidæ, Ptychoparidæ, Conocoryphidæ, Paradoxidæ, Phacopidæ, and Mesonacidæ)

Abstract

I. Introduction. The above-named species of Olenid was first recorded in 1874 by Callaway from the Shineton Shales of Shropshire, which he then claimed as of Tremadoc age. It was described from imperfect material as a Conocoryphe, which name in 1877 he corrected to Olenus, giving at the same time a description of the entire trilobite. A specimen for the first time exhibiting dorsal spines was in 1900 described by Dr. H. H. Thomas under a new name, Olenus mitchinsoni. An additional occurrence of the species was in 1905 recorded by Prof. W. G. Fearnsides under the name of ‘Parabolinella salteri’. It occurred in the upper part of his Nant-ddu or Bellerophon Beds, which follow next above the Dictyonema Band north-west of Arenig Fawr, and it is there accompanied by Asaphellus homphrayi Salter and Parabolinella triarthrus (Callaway), which species are also associated with it at Shineton. In 1907 I briefly described its development, and assigned it to a new sub-genus ‘Leptoplastides’. Finally, Mr. P. Lake redescribed it in 1919, under the name Leptoplastus salteri. It is now many years since, at Prof. Charles Lapworth's suggestion, I undertook the study of the trilobite fauna of the Shineton Shale. The first collection studied was that collected by Mr. Rhodes for H.M. geological Survey, and lent to Prof. Lapworth by the then Director-General. Among these were several slabs covered with minute examples of the fossil in question, in different stages of development; and the early stages seemed so strange and unexpected that a special study