14 DEVONIAN FAUNA. 



touching and with short necks behind, surrounded by a flat limb, very narrow at 

 the back, but dilate at the sides, which is slightly excavate and reaches some distance 

 in front of the spheres. Anterior side straight, bearing a spine pointing forward 

 at the corner. Border bearing sixteen large, and apparently very long spines. 



Size of Pygidium. — 14 mm. in width, 7 mm. in length. 



Locality. — Lummaton. A single specimen in my collection. 



Remarks. — This little fossil for a long time caused me much perplexity. It is 

 in an imperfect and obscure condition, and it was only by reference to foreign 

 species that its true character could be made out. However, it has proved to be 

 extremely similar to Arges radiatus, Goldf., 1 differing, nevertheless, as far as can 

 be ascertained, in various particulars. It is decidedly narrower, the excavate sides 

 come at least twice as far forward as the two spheres, leading to the impression 

 that the latter are comparatively smaller in the English species ; the spines too, in 

 our fossil, seem decidedly stouter and longer, and though they are in all cases 

 broken, so that it is impossible to say how much longer they were, probably the 

 difference was great, as they show no signs of tapering ; lastly, there seems to be 

 a spine situate on the front margin, close to the lateral angle. The German 

 species is also described by Barrande 2 from his Etages E. and F. of Bohemia, where 

 he states it to be very rare. His figure exactly corresponds with that of Goldfuss, 

 except that he gives the first segment of the axis, which is not seen in our, or the 

 German, fossil. 



Acidaspis radiata is the only fossil that could be mistaken for our species, 

 the tails of Acidaspis generally being formed on a very different plan. But it is 

 pointed out by Barrande, and it is interesting to notice, that there are approxima- 

 tions to the bilobed form in one or two other species. In Acidaspis PortlocJci, 3 

 Barr., from Etage E, are two large and definite swellings behind the last segment. 

 In A. vesiculosa, 4, Beyr., Etage F, similar swellings are visible, but of a much smaller 

 size ; in A. minuta, 5 Barr., Etage E, they are reduced to two large tubercles 

 upon the last segment ; and in A. Leonhardi, Barr., 6 Etage E, and A. Dufrenoyi, 7 

 Barr., Etage E, they become merely small granules or dots. Under these circum- 

 stances it becomes a question how far these variations are to be regarded as specific 

 only, and, if a distinct genus is to be formed, how many adjacent species it should 

 conclude. This question can be ultimately decided only when the perfect animal 

 is found, but in the meantime we may consider A. pilata, A. radiata, and A. 

 PortlocJci to belong to a single group. 



■ 1843, Goldf., ' Neues Jalirb. fur Min.,' 1843, p. 544, pi. iv, fig. 1. 



2 1852, Barr., "Syst. Sil. Boh.,' vol. i, p. 749, pi. xxxix, fig. 24. 



3 Ibid., p. 744, pi. xxxviii, fig. 10—12. 



4 Ibid., p. 715, pi. xxxviii, fig. 13—21. 5 Ibid., p. 729, pi. xxxvii, figs. 18—22. 

 G Ibid., p. 720, pi. xxxvii, figs. 1—11. ? ibid., p. 741, pi. xxxviii, fig. 25, 26. 



