Crustacea. | LOWER PALAZOZOIC ARTICULATA. 171 
IsoTELUS (Basilicus) TYRANNUS (Murch. Sp.) 
Ref. and Syn.—Asaphus tyrannus Murch. Sil. Syst. t. 25. f. 1. 
Sp. Ch.—Cephalic shield semicircular, about twice as wide as long, length equalling the thoracic and 
three first pygidial segments; glabella obscurely pyriform, convex, not reaching the front margin, with an 
obscure tubercle on the neck and an oval swelling on each side between the neck and the eye-lobe; eyes with 
very minute lenses, narrow, crescent-shaped ; ¢horaa, axal as wide as the lateral lobes ; pygidiwm elevated, one- 
third longer than the head, obtusely triangular, one-fifth wider than long, axis less than half the width of the 
lateral lobes, moderately convex, of about fifteen distinct and many minute segments, sides with about thirteen 
strong, equal, rounded ribs reaching about two-thirds to the margin, the upper ones reaching nearest to the 
margin, and nearly at right-angles to the axis, becoming gradually more oblique, and reaching a shorter way as 
they approach the extremity, leaving a steep, concave, smooth margin, one-third the width of the pleurz all 
round. Average length of pygidium two to three inches. 
Position and Locality—Llandeilo, Caermarthenshire; Craig y Glyn, near Llanarmon Fach, E. of the 
Berwyns; Rhiwlas, N. of Bala Lake, Merionethshire; Goldengrove, Llandeilo; Nant yr Arian; Tre Gil, S. 
of Llandeilo. 
Genus. ILLAZNUS (Dal.) 
Gen. Char.—Cephalic and caudal shields forming two nearly equal and similar portions of hemispheroids, 
the articular margin of each being truncated, the base of the glabella and caudal axis only indicated in the 
respective shields by two short axal sulci; nine or ten body-segments, destitute of pleural grooves, and 
with long, very narrow, nearly obsolete facets. yes and eye-line nearly as in Asaphus. 
There are three British subgenera:—lst J7/aenus, having ten body-rings and the lateral cephalic angles 
rounded. 2nd Bumastus (Murch.), differing only in having no trilobation of the body. 38rd Dysplanus 
(Burm.), having the lateral cephalic angles produced backwards into spines, and only nine body-rings, 
otherwise like Z//cenus. 
Subgenus. TiumNus (Dal.) restricted. 
Gen. Char.—Body subcylindrical, with equal rounded ends ; cephalic shield subhemispherical; glabella only 
defined by two short, parallel, unconnected longitudinal furrows at the posterior margin; eyes smooth, small, 
semicircular, placed very far apart on the sides of the head; eye-ines subparallel, longitudinal, cutting the 
posterior margin near the middle of the cheeks, forming the small eye-lobe over the eye and extending 
thence to two widely-distant points of the anterior margin, where they are connected by a transverse 
suture; thorax three-lobed, of ten equal segments; pygidiwm resembling the buckler in size and shape; 
axis only indicated at the anterior margin by two short parallel unconnected furrows. 
Intanus Davisu (Sa/t.) Pl. 1. G. fig. 36. 
Ref.—Mem. Geol. Sury. Dec. 2. t. 2. 
Sp. Ch—Head, thorax, and pygidium, of nearly equal length, gibbous; cephalic shield semioval, length 
rather more than two-thirds the width, very gibbous, the front nearly vertical; axal furrows sigmoid, half 
the length of the head, slightly nearer in front than at base; the included space about one-fourth wider 
than the cheeks; eyes rather more than their own length in advance of the posterior margin, and half- 
way between the axal furrows and the margin; thoracic segments with the axal and lateral portions of 
about equal width, deflexion from the knee close to the axis in the anterior segments, gradually removing 
to one third of the width in the posterior ones; pygidiwm semioval, length two-thirds the width, moderately 
convex, the outer third of the circumference rather abruptly sloping to the margin, axal furrows barely 
indenting the margin. Surface smooth. Average length of head nine lines. 
22 
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