SPH^REXOCHUS MIRUS. 47 



has been obtained recently by Mrs. Gray, illustrates these differences well. The 

 pleurae are simple, rounded and cylindrical, and devoid of any furrow on their 

 surface or transverse constriction, and they taper into free rounded slightly recurved 

 pointed ends; in Gli-. davif rolls'^ the inner portion is separated from the outer by a 

 sharp constriction and bears a strong diagonal furrow upon it. In the case of the 

 pygidium we observe that there is no enlargement of the first and second pleuras 

 as in Ch. clavifrons. Indeed the whole type of pygidium more recalls certain 

 Bohemian species, e. g. Ch. glohosits, Barrande,- and Gh. pectlnifer, Barrande.^ 



It does not seem possible to include Gh. octolobatus in the sub-genus G i/rtometopus 

 to which Gh. clavifrons belongs. 



The smooth unfurrowed simple pleura recall those of Sphsevexochiis and some 

 members of Pseudosphserexochuii, and the pygidium and glabella show also features 

 characteristic of the latter sub-genus. As mentioned by me on a previous occasion,* 

 Gh. odolohatus possesses a combination of characters indicating wide-spread and 

 complex affinities, and it does not seem possible to include it in any of the groups 

 or sub-genera of Ghelrurns which have been so far defined. 



Sphserexochus mirus, Beyrich, var. nov. balclatchiensis. Plate VIII, fig. 2. 



1906. Sphxrexochns mirus, Eeed, op. cit., pt. iii, p. 151. 



Specific Characters. — Head-shield as in Sph. minis. Thoracic segments with 

 similar characters. Pygidium with conical axis ending behind in acutely pointed 

 tip ; axis composed of two complete convex rings and a third incompletely 

 separated, followed by a pointed triangular terminal piece ; lateral lobes composed 

 of three pairs of obtuse lobate convex pleurae with free slightly reflexed bluntly 

 pointed ends, the third pair sub-parallel, embracing terminal piece of axis and 

 extending behind it. Surface finely granulated. 

 Dimensions. — 



Length of pygidium .... 18*0 mm. 



,, of axis ..... 15*5 ,, 

 Width of pygidium . . . . . 29*0 ,, 



,, of axis ..... 10*5 ,, 

 Eemarls. — Our previous ignorance of the pygidium prevented the separation 

 of this Lower Ordovician form from the Wenlock species. The head-shields 

 appear to be indistinguishable, as the author remarked in 1906. But the segmen- 

 tation of the pygidial axis is different, only two rings and a terminal piece being 



1 Schmidt, ' Eev. Ostbalt. Silur. Trilob.,' pt. i, p. 153, pi. viii, figs. 4—6 ; pi. xvi, figs. 7—12. 



2 Barrande, 'Syst. Silur. Bolieme,' vol. i, pi. xxxv, figs. 1, 2. 

 ^ Barrande, ibid., Suppl. p. 93, pi. iv, figs. IG — 20. 



* Eeed, ' Geol. Mag.' [4], vol. iii (1896), p. 163. 



