162 F. R. Cotcper Reed — Evolution of Cheivurus. 



of long spinose pleurte : the third pair is very short and rudimentary, 

 and of the fourth pair there is no trace. It is a noteworthy point 

 that tbe pygidia of this subgenus are, as a rule, almost identical with 

 those in the group of Cheirurus (sens, str.), with three equal pleurae 

 with free ends (vide antea), and do not show the enlargement of the 

 first pair, as in the Russian Cheirurus (sem. str.). 



The fullest development of the special features of this subgenus 

 is found in the Devonian species, such as Ch. gibbus (Beyr.). The 

 English forms are Ch. Pengellii (Whidborne) and Ch. Stembergi 

 (Boeck), from the Middle Devonian limestones. No members of the 

 genus are known above tbe Devonian, so that stratigraphically, as 

 well as in points of anatomical structure, Crotalocephalus is the 

 highest in the series. 



Returning now to the point where the main stem gave off the 

 Crotalocephalus and Pseudosphmrexochus brandies, we find the latter 

 branch arising from the neighbourhood of Ch. elaviger (Beyr.) and 

 leading first to the subgenus Nieszlcoivslcia, which, by the number of its 

 pleuras (12) and the absence of a constriction on them, as well as by 

 the presence of a row of puncta instead of a furrow, and the weakness 

 of the fulcrum, shows the persistence of certain primitive characters, 

 and its close connection with Eccoptocheile. But in tbe pygidiuin 

 we find a very considerable amount of modification, and in fact 

 Nieszlcoivslcia shows a more specialized pygidium than any member 

 of the whole genus. This indicates that the evolution of the different 

 parts of the body did not proceed at equal rates ; and we have seen 

 throughout that the pygidium is the part which suffers the most 

 diverse, irregular, and non-consecutive changes, being in one case of 

 a reversionary type, while in a closely-allied species it has characters 

 far in advance of the amount of specialization shown by the rest of 

 the body. 



In Nieszlcoivslcia the pygidium possesses only two pairs of pleura?, 

 and these end in free points ; the anterior pair is of huge size and 

 usually three or four times as large and long as the posterior pair, 

 which it completely embraces laterally. 



The head-shield of this subgenus also shows a remarkable kind of 

 development, for the glabella is inflated posteriorly and rises into 

 a hump, or is produced into a spine. The side-furrows resemble 

 those of its immediate predecessors, and the very forward position of 

 the posterior branch of the facial suture, and the small free cheek, 

 likewise recall the characters of Eccoptocheile. The type of the 

 subgenus is Ch. (N.) cephaloceros. and Angelin's Cyrtometopus tnmidvs 

 and Cyrfo. gibbus also belong to it. Nieszlcoivslcia occurs in the lower 

 part of the Ordovician of Russia and Sweden. 



The last stage of this line of development is shown by the sub- 

 genus Pseudosphcerexochus, which in its glabella resembles Sphar- 

 exochus, though the basal lobe is not so sh-ongly or so completely 

 marked off. The posterior branch of the facial suture cuts the outer 

 margin of the head-shield such a short way in front of the genal 

 angle that it prepares us for the state of things in Spharexochus, 

 where the suture cuts the genal angle itself. The pleural, with 



