PHACOPS. 53 



as that given in PI. Ill, fig. 17. It is from Malvern, as is also fig. 16, with the same 

 prominent ornament. 



Here, in this full-grown and very large specimen, it is shown that not only are the 

 tubercles more than superficial ornaments, for they are distinctly marked out by shallow 

 oval pits within the crust, but that they are actually far more numerous within, a 

 pair occurring in fig. 11 on every joint of the axis. It will be observed that they occur in 

 the adult in precisely the same relative position as in the young, viz., externally at intervals 

 on the axis, and on the second, third, fifth, sixth, and seventh lateral ribs, or rather on the 

 intermediate duplicating furrows of these ribs. 



Their position on the side ribs is remarkable. The first pair a, occur on the second rib 

 about half way out ; the next pair closer in to the axis. In like manner those on the fifth 

 segment b, are far out, and those on the two succeeding ribs gradually close in. This 

 regularity of position should be taken into account, as it indicates that there is some 

 structural importance in them.* 



Var. /3, TUBKRCULATO-CAUDATUS. PI. IV, fig. 1 . 



Phacops tuberculato-caudatus. Murchison, Sil. System, pi. vii, fig. 8 b, 1837. 

 — caudatus. Siluria, 2nd ed., pi. xviii, fig. 1, 1839. 



Form broad-ovate. Head decidedly angulated in front. Glabella not so wide as the 

 cheeks. Tail triangular, pointed, and a little produced. 



Comparing this with the ordinary variety, we find that the principal differences reside 

 in the generally broader form, the length being to the breadth about as 6 to 4, or rather 

 longer. This excludes the extreme mucro, which is broken off in our largest specimen. 



Besides this, the margin of the head is broader, and more sharply distinguished from 

 the head by a deep marginal furrow. The glabella is somewhat narrower and shorter, 

 with few large tubercles. The eyes quite as large and conspicuous as in the larger-eyed 

 forms of the ordinary variety, and which I suppose to be the male ( $ ), and the front is 

 more angular; it is produced, but not nearly to the extent of the anterior mucro in 

 the next species, P. longicaudatus. 



The thorax has remarkably large tubercles on the axis, and a central pair on the second 

 ring, and the pleurae on this specimen are tumid at their origin. 



The axis of the tail is short and rather flat, and the side lobes have seven or eight very 

 prominent ribs ; on the fourth, fifth, and sixth of which the remarkable gland-like tubercles 

 show strongly. The upper one on the first rib is not so prominent. The tail is strongly 

 margined, and the mucro (broken off) must from its shape have been short and rather 

 suddenly acuminate. 



* They are occasionally even more numerous than here indicated, occurring as tubercles on several of 

 the lateral segments of the body-rings, as well as on those of the tail (specimen in Mr. Edgell's collection), 

 see also PI. Ill, fig. 23. 



