88 
known; sagittal length approximately 60% of maximum width; axis 
with width about 70% of sagittal length; first three pleural furrows 
deeply incised; interpleural furrows nearly obscure; articulating 
half-ring set off from axis by prominent ring furrow; posterior axial 
rings and ring furrows difficult to discern; subdued but definite 
pygidial border developed around margin; posterior margin evenly 
arcuate; lateral and posterior aspects of cranidium defined as 
subvertical wall (PI. 4, fig. 7b), which is turned out again in a second, 
ventral, marginal rim; doublure relatively broad. 
DISCUSSION. We have not formally named this species because the 
two best cranidia (Pl. 4, figs 8, 11) were lost or broken following 
photography, and there are no replacement specimens suitable to 
serve as types. . 
The Tourmakeady species is obviously related to previously 
described species of Jsocolus, indicated particularly by the slot-like 
glabellar furrows that are greatly shallowed adaxial to the contact 
with the axial furrow. However, the laterally concave glabella of the 
Irish form is unique in the entire group. Other autapomorphic 
features include the apparently lacking dorsal sculpture, versus 
prominent raised lines in other species, the significantly broader 
posterior fixigena, and the posterolaterally directed, versus 
subtransverse, glabellar furrows. 
The ventral morphology of the /socolus pygidium has not previ- 
ously been described. The Irish material reveals the unexpected 
presence of a vertically oriented wall beneath what appears dorsally 
to be the pygidial margin. This ‘wall’ is flared ventrally into a 
second, ventral rim (PI. 4, fig. 7b). Reference to articulated material 
(Whittington 1956, 1963) indicates the larger, dorsally placed rim is 
almost certainly the true pygidial margin, and not analogous to the 
fulcral processes, rim, or spines often seen in groups like 
Entomaspidae Ulrich in Bridge, 1931 (e.g., Ludvigsen & Westrop in 
Ludvigsen ef al., 1989). The ventral wall and rim may therefore be 
doublural in origin. 
Family CATILLICEPHALIDAE Raymond, 1937 
DISCUSSION. See remarks under discussion of Isocolidae above. 
Catillicephalid gen. et sp. nov. PI. 3, fig. 4; Pl. 4, figs 1-6 
MATERIAL. Assigned specimens It. 25966, 25977-25982. 
DISCUSSION. The forward-expanding glabella, very short 
preglabellar field, tiny palpebral lobe set near to the axial furrow, 
anterior eye position, broad posterior fixigena, and distally elon- 
gated (exsag.) posterior cranidial border all indicate relationship of 
this unusual Irish species to Sunwaptan-Ibexian forms presently 
assigned to Catillicephalidae. The size and attitude of the posterior 
border as well as the position and inclination of S1, position and size 
of the palpebral lobe, and size of the cephalic border are further 
similarities to the stratigraphically nearest species, ie. Distazeris 
adoceta Ingham (in Ingham et al., 1986), from the Highland Border 
Complex of Scotland (Pratt 1992: 73 has criticized the generic 
assignment of this species). 
PLATE 5 
J.M. ADRAIN AND R.A. FORTEY 
The Irish species possesses a feature that distinguishes it from all 
related taxa, and apparently from all other trilobites: the develop- 
ment at the junction of the axial, posterior border, and occipital 
furrows of an exoskeletal hood that encloses a cone-shaped space 
opening laterally. The homology of this structure is very difficult to 
determine. The posterior border furrow runs directly into it, and 
appears to terminate beneath the hood (PI. 4, fig. 2d). The occipital 
furrow meets the axial furrow on the adaxial side of the hood, and the 
junction of these furrows continues to circumscribe the hood 
posteriorly (PI. 4, fig. 2b). 
Family ILLAENIDAE Hawle & Corda, 1847 
Genus ILLAENUS Dalman, 1827 
TYPE SPECIES. Entomostracites crassicauda Wahlenberg, 1818, p. 
27, from the Llandeilo of Fyacka, Dalarna, Sweden; by subsequent 
designation of Pictet (1854: 515). 
Illaenus weaveri Reed in Gardiner & Reynolds, 1909 
Pl. 2, figs 7, 10-12; Pl. 3, figs 6-8, 10, 11; 
Pl. 5, figs 1-11; Pl. 6, figs 1-12 
1909  Illaenus weaveri Reed in Gardiner & Reynolds: 142, pl. 6, 
figs 1-3. 
1910 Illaenus weaveri Reed in Gardiner & Reynolds: 272. 
1945 Illaenus weaveri Reed: 63. 
1968  Illaenus weaveri Reed; Whittington: 56. 
1988  Illaenus weaveri Reed; Morris: 115. 
DIAGNOSIS. Terrace lines on rear of cranidium, restricted to ante- 
rior part of librigenal field; librigenal flange only moderately 
developed; vincular furrow only impressed posteriorly on librigenal 
doublure; pygidium with sagittal length 55-60% of maximum width. 
MATERIAL. Lectotype, selected here, SM A10387, pygidium (PI. 
2, fig. 7), original of Reed in Gardiner & Reynolds (1909, pl. 6, fig. 
2); paralectotype SM A10316, cranidium and right librigena (PI. 2, 
fig. 10); topotypes It. 25959, 25960, 25967-25971, 25992-26014. 
DESCRIPTION. Due to the varying amounts and vectors of distor- 
tion of much of the available material, measured ratios are. 
approximate at best. The large pygidium of Pl. 6, fig. 7, is considered 
to be nearly undistorted, and pygidial ratios are based upon this 
specimen. 
Cranidium with length (sag., measured in sagittal profile) 75— 
88% of maximum width across midlength (exsag.) of palpebral 
lobes; maximum anterior width about 90% of width across palpebral 
lobes; posterior sections of facial sutures declined nearly vertically 
when palpebral lobe is oriented in horizontal plane, diverging sharply 
posteriorly, nearly transverse; anterior sections of facial suture 
declined at about 30 degrees from horizontal when palpebral lobe is 
oriented in horizontal plane, slightly anteriorly divergent immedi- 
ately in front of palpebral lobe, then forming even, laterally convex 
arc to converge near anterior margin; palpebral lobes relatively large 
and elongate; axial furrow strongly effaced, impressed only 
posteriorly, behind midlength of palpebral lobe; entire cranidium 
Figs 1-11 //laenus weaveri Reed in Gardiner & Reynolds, 1909 1a-b, It. 25992, cranidium and left librigena, anterior and left lateral views, x10. 2a-b, It. 
25993, cranidium and right librigena oblique and anterior views, x4.5. 3a-e, It. 25994, cranidium, dorsal, anterior, and left lateral views, x7.5. 4a-e, It. 
25995, cranidium, dorsal, anterior, and right lateral views, x10. 5a-c, It. 25996, cranidium, dorsal, anterior, and left lateral views, x15. 6, It. 25998, 
rostral plate, ventral view, x10. 7a-b, It. 25997, rostral plate, ventral and dorsal views, x7.5. 8a-b, It. 25999, rostral plate, posterior and ventral views, 
x10. 9a-b, It. 26000, cranidium, dorsal and anterior views, x15. 10a-c, It. 26001, cranidium, dorsal, anterior, and left lateral views, x15. 11, It. 26002, 
cranidium, ventral view, x10. 
