120 F. R. Coirper Reed — Evolution of Clieirurus. 



better known Eccoptoclieile Sedgmcki (M'Coy), which occurs in the 

 Upper Llandeilo of Wales. The latter species shows on each of its 

 twelve thoracic pleurae a longitudinal groove extending fully three- 

 fourths of the whole length of the pleura, and furnished with a row 

 of puncta lying in it. The pygidiuiu, while showing four segments 

 on the axis, has only three pairs of pleura?, indicating a condensation 

 and abbreviation of this part of the body. The pygidial pleura? are 

 closely similar to those of the thorax, preserving the longitudinal 

 groove and row of puncta. M'Coy's original figured specimens of 

 this species are in the Woodwardian Museum, as well as those of 

 Ch. (Anach.) Frederick The presence of the row of puncta in the 

 longitudinal pleural groove, and the shortening of the latter, are 

 characters intermediate between those of Ch. Frederici and the next 

 group of species. This group, which represents the next stage in 

 the development of the genus, includes several Bohemian species 

 (e.g. Ch. claviger, Barr., Ch. pater, Barr., etc.). 1 The longitudinal 

 pleural groove is reduced in length so as to furrow only about half the 

 pleura, which can therefore be considered to consist of two portions 

 — an outer ungrooved portion and an inner grooved portion. A row 

 of puncta is present in the groove. The Bohemian forms, however, 

 have curiously only ten thoracic segments, which probably points to 

 an arresting of the process of segmentation at a pre-adult stage. 

 The preservation of an adolescent character in the adult is, of course, 

 well known in many groups of organisms, and I do not attach any 

 subgeneric importance to it. The pygidium resembles in the 

 number of the pleura? and condition of the axial portion that of 

 E. Sedgivicki, but the greater dissimilarity of the pygidial to the 

 thoracic pleurae points to a further specialization of the pygidium. 

 The head-shield demands no special notice, since its characters 

 are essentially the same as those in E. Sedgicichi. Ch. claviger 

 occurs in Dd 3 and Dd 4, but principally in the latter, which 

 corresponds with the lower part of our Bala. The allied 

 Ch. scuticaitda (Barr.), which shows a slightly greater reduction 

 in length of the pleural groove and line of puncta, has the 

 atavistic character in the pygidium of four pairs of pleura?, which, 

 nevertheless, by their form, etc., show some specialization. A 

 reversion to more primitive and ancestral conditions in the case of 

 the pygidium is by no means rare in this genus, as we shall see. 

 Ch. scuiicauda occurs in Dd 3 of Bohemia. Ch. pectinifer, Barr., of 

 Dd 5. with its Cyrtometopian head and reduced pleural groove, 

 appears to belong to about this stage. 



The species which comes next in the series is Ch. tumescens (Barr.). 

 It seems to be closely related to Ch. scuticaitda, for it possesses a 

 very similar pygidium, and also commences on the same horizon 

 (Dd3), though ranging up to Dd 5. In this species also only ten 

 thoracic segments are present. The chief advance is shown in the 

 constriction which occurs between the outer and inner portions of 



1 It is remarkable that this group, which includes the earliest members of the 

 genus occurring in the Bohemian basin, has not yet been found in northern Europe 

 (see sequel). 



