F. R. Cowper Reed — Evolution of Cheirurus. 119 



are triangular and small, since the posterior branch of the facial 

 suture cuts the outer margin far forward. The genal angles are 

 produced into long divergent horns. The pleurae have a weak 

 fulcrum, and while the inner portion has a longitudinal row of 

 puucta, the outer portion is smooth and produced into a sword- 

 shaped point. The pygidium is always four-pointed, and the 

 posterior pair of points is embraced by the larger anterior pair. 



Group 2 of Section II is formed by Pseudosphcerexochus, the type 

 of which is P. hemicranium (Kut. sp.). The glabella resembles that 

 of Sphcerexochus ; the first side-furrow stands almost vertical to the 

 axal furrow; the third side-furrow, which is much stronger than 

 the two first, is also almost vertical to the axal furrow for part of its 

 length, but then bends back and becomes weaker, reaching the neck- 

 furrow only as a faint groove. The free cheeks are large and form 

 almost the whole side of the head-shield, for the outer limb of the 

 facial suture cuts the margin only a short distance before the genal 

 angle. (In Spkcerexochus this outer limb reaches the margin at the 

 genal angle.) Th£ pleurae have a distinct fulcrum ; the inner part 

 of each pleura is swollen, and possesses either an almost obsolete 

 row of puncta or no punota or groove at all ; the outer part is long 

 and conically pointed. The pygidium has four pairs of pleurae with 

 free pointed ends. 



Eccoptocheile is not described by Schmidt, since it does not occur 

 in the Silurian of the East Baltic provinces ; but he alludes to the 

 English species E. Sedgwiclci (M'Coy) as belonging to this section. 



The earliest type of Cheirurus in Britain occurs in the Upper 

 Tremadoc of Portmadoc, and was placed by Salter in the subgenus 

 Eccoptocheile. This species [Ch. (E.) Frederici, Salter], though 

 undoubtedly closely allied to the members of Section II in Schmidt's 

 classification, yet shows some important points of difference to the 

 typical Eccoptocheile. The Tremadoc form has eleven or twelve 1 

 thoracic segments, each of which is deeply grooved along its middle 

 almost to its free end. There is no row of puucta in the groove. 

 The great length of the groove, the absence of puncta in it, the 

 loosely-built structure of the thorax, and the close similarity of the 

 pygidial to the thoracic segments, look like primitive characters. 

 1 am inclined to think that this species is sufficiently distinct from 

 the typical Eccoptocheile to warrant the creation of a new subgenus 

 for its reception. This subgenus may be termed Anacheirurus. 2 



The next species as we ascend the stratigraphical series is Ch. 

 (Eccopto.) pectinatus (Salter MS.), of the Lower Llandeilo or Arenig 

 of Shelve ; the only specimens with which I am acquainted are in 

 the Museum of Practical Geology, Jermyn Street, and no description 

 of its characters has been published. It seems closely allied to the 



1 The type specimens on which the species was founded are not sufficiently well 

 preserved tor one to be sure of the number of segments — a point Salter notices. 



2 The pygidium with seven segments ascribed to this species (Salter, Mou. Brit. 

 Trilob. Pal. Sac., pi. v, fig. 21, p. 76) probably belongs to a distinct species, or even 

 subgenus ; but our ignorance of the other parts of the trilobite to which it belonged 

 prevents us coming to a decision about its exact position. 



