138 ' SILURIAN TRILOBITES. 



Generic Characier. — Ovate, greatly depressed ; head with short spines. Glabella 

 widely clavate, the axal-furrows nearly parallel below, and sometimes obsolete above. Eyes 

 large, depressed, anterior in one subgenus, subcentral in the other. Facial suture marginal 

 in front, nearly vertical posteriorly. Labrum pointed? Pleurae falcate. Tail large, 

 fan-shaped, with a shortened axis, and few lateral furrows ; the caudal fascia occupies a 

 large part of the limb beneath. 



Subgenera. — 1. Homalopteon} Salter, 1865. Glabella with complete axal furrows, 

 widely clavate above, more or less distinctly lobed transversely by four transverse furrows. 

 Eyes anterior. Pleurae with remote fulcrum, grooved, but scarcely facetted. Tail with 

 a distinct short axis of several rings ; the sides few-ribbed. Manye. — Llandeilo Flags. 



2. Barrandia, M'Coy, 1849. Glabella with incomplete axal-furrows and no distinct 

 lobes. Eyes subcentral. Pleurae falcate, with a fulcrum close to the axis, grooved, not 

 facetted. Tail with a short ribless axis and smooth sides, the strong articular furrow only 

 present. Ranc/e. — Llandeilo Elags. 



Subgenus — Homalopteon. 

 Barrandia (Homalopteon) Portlockii, Salter. PI. XIX, figs. 6 — 10. 



AsAPHUS DiLATATUS, Portlock. Gcol. Report of Tyrone, &c. ; p. 293, pi. xxiv, fig. 2 



(not o( Daltnan) 1832. 

 Ogygia dilatata, Suiter and Phillips. Mem. Geol. Survey, vol. ii, pt. I, p. 239, 



1848. 

 — PoETLOCKii, Salter. Decades Geol. Survey, No. 2, pi. vii, figs. 1, 2, 6, 7 



(not figs. 3 — .5, for which see next species), 1849. 



-S. {Homal.) ovali-oblovga, 2)^ uncialis, capitis margine angusto. Glabella genis latior, 

 antice gibbosior, lobata, lobis utrinqiie 4, distinctis. Thorax axe lato nodosa ; pleuris per- 

 sulcatis. Cauda axe lato h-annulato, apice acuto ; lateribus semicostatis, costis ^-radiatis 

 divisis subrectis. 



The length of this, the largest of the three species known, is full four inches, judging 

 by fig. 8, which is from the original specimen in Portlock's plate. It is nearly flat, ovate, 

 widest in front ; the head forming rather more than a semicircle, and considerably wider 

 than the tail, but about as long as the thorax. The glabella is as wide as the cheeks below, 

 and separated by narrow but distinct and complete axal-furrows from them. It widens and 

 overhangs the eyes above, and is there somewhat abruptly bent down, the margin being over- 



cloaely allied to Ogxjgia and Stygina in habit. The Bronteidee have, on the contrary, a thick calcareous 

 shell, and often a highly convex, almost gibbous form, while the axis is so greatly reduced as to be almost 

 obliterated. I shall return to the Bronteidse as soon as the Asaphidse are completed. It is impossible 

 to take the affinities in a linear form, because that is not the order of nature. 



1 6//a\os, platmm, Trreov, Att. for vtvov, ^abellum ; from the flat, radiated character of the tail. 



