70 BRITISH CAMBRIAN TRILOBITES. 



Triarthrus shinetonensis, but that the specimens do not belong to the latter species 

 is clearly shown by the breadth of the frontal limb. 



A young form, presumably belonging to this species, is shown in Plate VII, fig. 

 12. It has, apparently, eight thoracic segments, but otherwise does not differ 

 very greatly from the adult. The head, unfortunately, is incomplete. 



Horizon and Locality. — Shineton Shales : Shineton. 



Genus TRIARTHRUS, Green. 



The type of this genus is Triarthrus beclci, Green, 1 and the prominent features of 

 that form are the large glabella reaching nearly to the anterior margin ; the position 

 of the eyes in the middle of the cheeks; the narrow free cheeks with the genal 

 angles rounded; the wide thoracic axis, considerably wider than the pleuras ; the 

 strongly marked fulcrum placed very near the axis ; the deep pleural grooves; and 

 the rather small tail with entire margin. The number of thoracic segments in the 

 fully developed adult is sixteen, and the axis of the tail shows about five segments. 



If the species described below and the Triarthrus spinosus of Billings are to be 

 included in the genus, the genal angles need not be rounded and the number of 

 segments in the thorax and tail may be considerably less than in T. beclci, but the 

 other characters will remain generic. 



Peltura is a closely allied form with similar pleurae, but it has the eyes placed 

 further forwards, the thorax consists of twelve segments, and the tail is serrate. 



1. Triarthrus shinetonensis, Raw. Plate VII, figs. 13 — 16. 



1908. Triarthrus shinetonensis, Raw, Rep. Brit. Assoc, for 1907, p. 512. 



Head semi-circular, slightly angulate in front, with the genal angles produced 

 into long spines. Glabella quadrate, a little longer than wide, slightly expanded 

 towards the front, with two pairs of glabellar furrows, both of which are sigmoid 

 and oblique, not quite reaching either the middle line or the axial groove ; neck- 

 furrow distinct, but barely reaching the axial groove ; neck-segment divided into 

 a large central portion and two antero-lateral lobes, with a median tubercle on the 

 central portion. Eye-lobes long, set rather forward and near to the glabella, with 

 an ocular ridge running towards the glabella. Facial suture probably marginal in 

 front, running backwards from the anterior margin to the eye and from the eye 



1 Excellent figures of this species, in all stages of development, are given by Walcott in Trans. 

 Albany Institute, vol. x (1883), p. 23, pi. ii, figs. 1 — 15 ; and reproductions of photographs both of the 

 dorsal and of the ventral surfaces, with the limbs, will be found in a paper by Beecher in Geo). Mag., 

 decade iv, vol. ix (1902), p. 152, pis. ix — xi. 



