ILL^NUS. 



203 



The tail is convex, but not uniformly so, the anterior part and the line of the axis 

 generally being rather flattened. Thence, towards the margin, it is bent down abruptly. 

 The breadth of the tail exceeds the length by about one half the latter ; the posterior 

 margin is a semicircle, and the anterior a shghtly wavy line (PI. XXX, fig. 7), on which 

 the broad axis is marked out by shallow indentations, and the angle at which the facet 

 starts a very obtuse one, so that the angles of the tail are scarcely more truncated 

 than in the "Barr Trilobite." There are but shght indications of very shallow axal 

 furrows, but the anterior lateral furrow of the tail beneath the fulcrum is quite distinct. 

 The fascia is slightly convex, and has coarse striae ; it is very narrow. The inner surface 

 of the tail rough, with small points or tubercles. 



Locality. — Caradoc or Bala Limestone, at Grug and Birds Hill, close to Llandeilo. 

 (Mus. P. Geol. and Mr. Lee.) Chair of Kildare (Mr. Wyatt-Edgell's Cabinet). 



Bumastus follows next in order of affinity; for though itz^mo.^"^ Edillanus and 

 lUcenopsis are in the trilobed group, they are so diflFerent structurally, on account of the 

 position of the eye, that we may well believe they will form distinct genera by-and-bye. 

 I shall not follow, therefore, the order of the numbers, but place the above-named two 

 sub-genera last. And we now come to the highest, if not the most typical' lUani, 

 \\z. the Upper Silurian forms of Bumastus, distinguished by the great soHdity and fine 

 sculpture of the shell or crust, the convexity of the body, the full number of body- 

 rings, largely developed eyes, &c. In this group the trilobation of the tail, all but lost in 

 some of those above described, is altogether absent, and that of the thorax nearly so. 



Subgenus 5. — Bumastus, Murch., 1837. 



Ili,.exus (Bumastus) Barriensis, Murchison. PI. XXVII, figs. 1 — 5. 



"A new species of Trilobite," F. Jukes. Ann. Mag. Nat. Hist., vol. ii, p. 42, figs. 8, 9, 

 10, 1829. "Probably Isotelus," J. Be C. Sowerby, 

 ib. p. 45. Silliman, Amer. Journ. of Science, vol. 

 xxiii, i, p. 203, 1833. 



Bumastus Barriensis, Murchison. Sil. Sjst. (description, but not figures), p. 656, 

 pi. vi bis, 1839. 

 — — Emmerich. Dissert., p. 33, 1839. 



NiLEUs? (Bumastus) Barriensis, Burmeister. Org. Tril., p. 120, 1843. Ib., Ray 

 edition, p. 104, 1846. 



^ The highest and lowest forms of a group are not typical : there is in most natural groups a central 

 form combining all the chief characters. At least that is my idea of type. Asaphus is typical of the 

 Aaaphidce ; it is not the highest form. Acaste is the typical Phacops ; and so forth. 



