Crustacea.] LOWER PALAXOZOIC ARTICULATA. 159 
Brongniart, Pl. 1. fig. 3. A., which has been referred to it by several authors (Mem. Geol. Surv., Burmeister, 
Fletcher, in Geol. Jour. Vol. VI., &c.), but to which Dr Buckland has applied the specific name tudercu- 
latus, which should be retained therefor. 
Position and Locality—Common in the Wenlock limestone, Dudley; sandstone of Pwllheli, Caernar- 
vonshire; Middleton Park, near Sedburgh. 
Sth Subfamily. ASAPHINA (J/Coy). 
Pleurze bent down at the end, and having distinct trigonal facets; thoracic segments eight to thirteen. 
This subfamily may be looked on as the type of the entire group, and contains the most perfectly 
organised trilobites. They all, I believe, have the power of rolling themselves into a ball, and they are 
the only trilobites having the triangular facets at the anterior part of the extremities of the pleure. 
They have a compact ovate form, and from the deflection of the margin are all of considerable depth. 
The subfamily contains the following British groups, to which the rarfk of families has been assigned 
by some writers, who, perhaps, magnified their importance from not studying them in connexion with the 
rest of the class Crustacea. I feel satisfied they are of only generic value, and each contains other groups 
hitherto called genera, but which seem to me only subgenera, and which I have grouped accordingly. 
Ist, Phacops; 2nd, Oalymene; 3rd, Trimerocephalus; 4th, Asaphus; 5th, IUenus; 6th, Forbesia; 7th, 
Phillipsia. 
Genus. PHACOPS (in a wider sense than Emmerich.) 
Gen. Char.—Head large, with the angles prolonged backwards into large spines ; glabella clavate, wider 
in front than at the base, and marked with three strong segmental furrows; eyes very large, reniform, 
with a coarsely granular, largely facetted inner cornea ; thoracic segments eleven; facial sutures cutting the 
lateral margin of the head in front of the angles. 
This genus contains the following British sub-genera :—Ist, Phacops; 2nd, Odontochile; 8rd, Portlockia ; 
4th, Chasmops. 
Sub-genus. Puacors (#m.) 
Gen. Char.~Ovate, contractile; cephalic shield semicircular, with a strongly-developed margin, ter- 
minating in spinous lateral angles; glabel/a clavate, broadly rounded in front, narrowed posteriorly, sides 
divided into three lobes, the anterior one on each side much the largest; eye-line curving round the 
glabella in front, bordering the small eyelid over each eye, and then passing from the posterior end of 
each eye direct to the lateral margin, which it cuts considerably in front of the lateral angle; eyes large, 
reniform, prominent, of many large lenses; thoraa of eleven segments; plewripedes subtruncate at their 
ends, facets large, triangular, strongly marked; pleural groove strong, slightly bent downwards at an angle 
on a level with the apex of the facet; pygidium semicircular or pointed, with eight to twelve joints in the 
axis, sides with about five strong duplex ribs, margin thickened, entire. 
PHACcops ALIFRONS (Salt. in Ap.) Pl. 1. G. fig. 12 to 14. 
Sp. Ch.—Cephalic shield about twice as wide as long, very gibbous; glabella narrow, clavate, elevated, 
sides nearly straight ; front obtusely rounded, projecting beyond the margin, with which, at the sides, it 
is confluent by a thick wing or horn-shaped extension on each side; (? lateral angles obtusely rounded), 
surface of the glabella coarsely tuberculated, lateral segmental furrows rather small, not very unequal in 
size; first lobe largest, first furrow directed obliquely backwards, the rest nearly transverse, with a slight 
inclination forwards, neck-segment about as thick as the last glabellar segment; cheeks steeply inclined 
from the glabella towards the lateral angle; surface with smaller granules than the glabella ; eyes small, 
reniform, close to the glabella. Length of head nine lines. 
The horn-like running out of the sides of the front of the glabella into the thickened margin of the 
cheeks gives a very peculiar character to the species. The pygidium supposed to belong to it is tumid, 
