AGNOSTUS EXARATUS. 7 



Head rounded, with narrow margin, axial and marginal furrows very deep. 

 Glabella convex, bilobed ; anterior lobe large, subquadrate, rounded in front ; pos- 

 terior lobe nearly parallel-sided, but slightly constricted in the middle, with ;i 

 small median tubercle ; basal lobes very small. Cheeks smooth, somewhat convex, 

 separated from each other by a deep furrow in front of the glabella. 



Thorax : axis with a wide, central, raised portion and small lateral tubercles. 



Tail rounded, widest posteriorly, somewhat longer than wide, axial and 

 marginal furrows very deep, margin rather wide. Axis convex, shows no trace 

 of segmentation, slightly constricted towards the anterior extremity, terminates 

 in a long sharp point; it bears a prominent median tubercle in the anterior third, 

 and the posterior extremity is somewhat depressed. Lateral lobes smooth, convex, 

 widen posteriorly, separated from each other by a deep furrow behind the axis. 

 Margin rather wide, expanded posteriorly. 



Head- and tail-shields about 5"5 mm. long, 5 mm. wide. 



The most striking characteristic of this form is the depth of the axial and 

 marginal furrows and of the groove behind the axis of the tail, and these 

 characters are not possessed to anything like the same extent by any other 

 British species. In compressed and distorted specimens, however, the strength 

 of the furrows is not always so conspicuous ; but in these cases the large rounded 

 lobe of the glabella and the rather long and very pointed unsegmented axis of the tail 

 are generally sufficient to distinguish the species. The axis of A. punctuosus in its 

 outline somewhat resembles that of A. exaratus, but the tail is easily distinguished by 

 its segmentation and by the shallowness of the axial and other furrows, the absence 

 of any furrow behind the axis (except a slight one in young forms), and by the 

 fact that the lateral lobes diminish in width posteriorly instead of increasing. In 

 perfect specimens there is not the slightest possibility of confounding the two. 



Gronwall has described the tail only, but some of our specimens show the head 

 and thoracic segments also. 



Synonymy. — Salter's Agnostus scutalis, as described by Hicks, includes at least 

 two distinct species. The description of the head corresponds fairly well with the 

 head of the species now described ; but the description of the tail is apparently 

 based on specimens of A. punctuosus. Hicks's fig. 9 is an imperfect representation 

 of a beautifully preserved tail of a young A. punctuosus ; fig. 10 is possibly a still 

 younger form of the same species (cf. Tullberg, ' Agnostus-artema,,' pi. i, fig. 5 c) ; 

 the original of fig. 12 is a rather poor specimen of the species under description ; 

 and the originals of figs. 11, 13, and 14 are unfortunately not known. Fig. 13, 

 however, is clearly a specimen of the species here described, and figs. 11 and 14 

 also probably belong to the same species. Several of the specimens in the 

 Sedgwick Museum, labelled Agnostus scutalis by Salter, certainly belong to 

 Gronwall's Agnostus exaratus. Owing to the confusion in the descriptions and 

 figures of A. scutalis, it is clear that Salter's name cannot stand. 



