PHACOPS. 



47 



Fig. 10. 



angustis acutis valde curvatis et cum tot lineis intermediis profundioribus ad apices confusis ; 

 margine angusto subundulato. 



From the number of authors who have described the Swedish species, it must be a 

 common one, and yet there is no good figure extant. Angelin's, the latest, is worse than 

 Dalman's, which appears to be the best, for Wahlenberg's figure is so very unlike the fossil 

 that, were it not for his description and statement of locality, it would be impossible to 

 recognise it. Brongniart has only copied Wahlenberg, who appears to have been chiefly 

 attentive to the course of the facial suture, and allowed his artist to take his own course. 



To make evident the distinctions between our British fossil (which is certainly very 

 nearly allied to the Scandinavian one), and the true P. mu- 

 cronatus, I subjoin a figure from one of Sir R. I. Murchi- 

 son's specimens. 



It will be seen readily that the head we here figure 

 has less regular and equal lobes than the British one, and 

 this is a character of high importance in the family. And 

 the tail, while it has a great resemblance, has not the 

 strongly curved and deeply impressed ends to the side fur- 

 rows, which give so conspicuous a character to our fossil. I 

 am justified, therefore, in placing a ? against the same : 

 indeed, had it not been already referred in the Decades 

 to P. mucronutus, I would have preferred keeping it dis- 

 tinct under the name P. appendicidatus. For the present I 

 leave it in doubt. 



Description. Our specimen of the head only shows a broad glabella with three equal 

 pairs of lobes, which are divided by nearly straight somewhat radiating furrows : the two 

 lower pair of lobes are linear and direct ; the upper somewhat clavate, but not triangular, 

 and separated by a slightly sinuous furrow from the wide transverse forehead-lobe. All 

 the lobes are tumid ; and a convex median ridge, of equal breadth throughout, divides the 

 lobes of one side from those of the other. A deep central pit in the cast occurs upon the 

 forehead-lobe, as in many species. 



Of the triangular tail we have more complete evidence, though even of this only a 

 fragment. 



The axis is broad and convex, of nine strong rings, and a terminal portion or appendix, 

 of which we do not know the length. While the axis is convex, the sides are very flat, 

 have the forward edge strongly curved, and are deeply grooved by seven arched duplicated 

 furrows. The backward curve of these near the margin is considerable, and the secondary 

 furrows are as strong or stronger than the primary, and coalesce with them at the tips in 

 deep grooves, so as to undulate the very narrow margin considerably. 



* " hoc. Nat. In stra'tis argillaceis E, Vestrogotbise, in montibus Alleberg et Mossberg, alibique.' : 

 Angelin, 'Palseont. Suecica,' part 1, 1852. 



Swedish specimen of P. mucronaius. 

 Lower Silurian; Alleberg Mountain, 

 Vestrogothia.* 



