£10 Dr, C. Davison—British Earthquakes. . 
One Upper Silurian species (Fig. 2), B. thysanopeltis, Barr.,' has as 
many as forty-five marginal spines on its caudal shield, while another 
detached pygidium shows thirty-three spines upon its border? (Fig. 3). 
B. (Thysanopeltis) speciosus, Corda, from the Lower Middle Devonian, 
has forty marginal spines around its caudal shield* (Fig. 4), while 
another Devonian form referred to by Barrande* has about twenty-two 
spines.° 
ie may, I think, safely conclude that Mr. Townshend Hall’s 
Gerolstein specimen is quite distinct from any other Devonian form. 
It may be characterized as having seven coalesced segments in its 
caudal shield, indicated by seven marginal spines, marking the lateral 
termination of the pleure, and by a single non-bifurcate stout median 
lobe, being a prolongation of the axis of the caudal shield, and 
terminated by a similar median spine upon its margin. The 
decorticated portion of the margin of the shield shows that the 
underside was etched by numerous fine parallel wavy lines, extending 
over even the lower surface of the marginal spines. 
I propose to designate this Gerolstein form as B. Halli, after my 
late friend Townshend M. Hall, who did so much good work in the 
Devonian rocks of North Devon. 
IX.—Tase CuHaracteristics oF British EartHquakes: A SuMMARY OF 
Twenty-one Years’ Work. 
By Cuarzes Davison, S8c.D., F.G.S. 
OR a detailed study of the earthquakes of any district, an interval 
of twenty-one years is too brief. Long-period variations of 
frequency cannot be established. We can form no satisfactory 
conception of the distribution of seismic energy in space, for some 
foci may le inactive for a much longer time, while others may 
continue in operation without apparent change. But, to ascertain 
the characteristic features of the earthquakes, to investigate their 
relations with those which precede and follow them, or to trace 
their connexion with the structure of the central districts, such an 
interval is possibly of sufficient length. In any case, the defects 
resulting from its brevity may be partly compensated by uniformity 
in treatment and in the methods of investigation. 
1 J. Barrande, Systéme Silurien du Centre de la Bohéme, pt. i, vol. i, 1852, 
Texte Crustacés: Trilobites, p. 843, pl. xlvij, fig. 6. 
2 Op. cit., pl. xlvii, figs. 11, 12. 
3 Kayser’s Zext-book of Comparative Geology (translated and edited by Philip 
Lake), p. 121, fig. 5, 1893; and Girich, Leitfossilien, Taf. xlvii, fig. 1. 
* See explanation to pl. xlvii, under fig. 12. 
> Barrande writes—‘‘ Bronteus acanthopeltis (Schnur) was recently discovered in 
the Eifel by Professor Schnur, of ‘'réves. It presents the nearest analogy with 
B. thysanopeltis. It is distinguished, however, by possessing less than half the 
number of spines around the pygidium. It only came to our knowledge by the 
kindness of Professor Schnur, who was so good as to send it to us at the moment 
when our text was going to press.’? [I cannot, I regret to say, find any figure of 
this species. | 
