Crusracea.] LOWER PALZZOZOIC ARTICULATA. 157 
portion about as wide as the axis, divided by a straight pleural groove into a large, very convex, anterior, 
tuberculated portion, and a narrow smooth posterior portion; beyond this the extremities are free and 
gently arched backwards and a little downwards for nearly as great a length; pygidiwm subrhomboidal, 
length about two-thirds the width, anterior margin very much arched, longer tian the posterior; axis 
narrow, conical, of about twenty narrow segments, of which only the first four reach entirely across; the 
others being confined to the sides, leave a smooth space down the middle, which is bordered by an obscure 
double row of tubercles (in the sandstone casts the axal segments all seem to go across); broad lateral lobes 
ot only four ribs, which are continuous at their origin with the four anterior axal segments; each bears 
an irregular row of tubercles, and terminates in a slightly projecting blunt extremity. Length of head 
about seven lines, of pygidium nine lines. 
In placing this species (which is the Cybele verrucosa of Lovén) in the genus Zethus, 1 have been obliged 
to propose a new specific name for it, as it is not the Z. verrucosus of Pander, and as I cannot agree with 
Volborth in referring it to the C. dellatula of Dalmann. Having in the article on the genus stated the 
reasons for uniting the genera Zethus of Pander and Atractopyge of Hawle and Corda, of which latter genus 
the present species formed the type, I have used the generic name of those authors to designate the species. 
Position and Locality—Not uncommon in the schists of Ravenstone Dale and Coniston, also in the 
sandstone of Alt yr Anker, Meifod, Montgomeryshire. 
Explanation of Figures—P\. 1. G. fig. 1. Part of head and part of thorax, natural size, from the 
schist of Ravenstone Dale——Fig. 2 and 3. Pygidium, natural size, from the slate of Coniston.— Fig. 4, 
Do. Magnified —Fig. 5. Part of head a little larger than natural—Fig. 5. Portion of fig. 5 magnified. 
ZETHUS SExcosTATUS (Salt. Sp.) Pl. 1. G. fig. 6 and 7. 
Ref. and Syn.—Cybele sexcostata. Salt. Mem. Geol. Surv. Vol. II. Pt. 1. t. 8. f. 9, 10. 
Sp. Ch—Pygidium twice as wide as long, subtriangular, or slightly rhomboidal from the backward 
curve of the front margin; axis narrow, cylindrical or very gradually tapering, of about twenty minute 
segments and an abruptly deflected tripartite smooth termination, and two obscure rows of minute tubercles, 
one pair on each segment; side lobes with six or seven strong obtuse ribs without tubercles, the first 
two pair nearly horizontal for the first third or halt of their length, then abruptly curved almost vertically 
downwards, the others bent down nearly from their origin, the ends projecting beyond the margin in the casts. 
Average length six lines. 
Position and Locality.—Abundant in the limestone of Rhiwlas N. of Bala Lake, Merionethshire, 
Llwyn y Ci, N.W. of Bala, Merionethshire. 
Explanation of Figures.—P\. 1. G. fig. 6. Pygidium, natural size, from the schists of Rhiwlas.—Fig. 7. 
Larger individual, same locality. 
ZETHUS VARIOLARIS (Brong. Sp.) 
Ref. and Syn.—Calymene variolaris? Brong. pars (t. 1. f. 3 B.) id Murch. Sil. Syst. t. 14. f. 1. 
Sp. Ch.—Cephalic shield rather more than twice as wide as long, obscurely trilobed in front by the 
projection of the broad front of the glabella, which is broadly clavate ; height and width of the cheeks about 
equal, lateral angles slightly produced ; eyes nearly marginal, rather nearer to the glabella than to the lateral 
angles; tubercles of the surface coarse, irregular, obtuse, each with a little pit on its summit, three on 
each side of the base of the glabella larger than the rest (? segmental) ; posterior margin of the head, 
and neck-segment smooth, and resembling one of the thoracic segments in character; thorax one-third 
longer than the glabella, axis nearly as wide as the pleur above, narrowing posteriorly, the few last 
segments having obtuse tubercular ends; plewrw with their posterior portion very small, anterior portion 
thickened and produced beyond the margin, anterior segments slightly bent down, and either horizontal 
or directed a little forwards at their extremities, posterior ones abruptly curved down about the middle 
or nearer the axis, nearly to right angles with their origin, and directed backwards, nearly smooth, with a 
