90 
vaulted, maximum sagittal curvature achieved slightly posterior 
to slightly anterior of palpebral lobes; anterior border and border 
furrow not evident; prominent terrace lines running subparallel to 
anterior margin, finer and more closely spaced near margin, coarser 
and more widely spaced dorsally; fine subparallel, transverse terrace 
lines developed across rear of glabella, occipital region, and rear of 
palpebral lobes, originating with close spacing laterally, spacing 
greater sagittally, forming ellipsoid pattern (Pl. 5, fig. 3a); occipital 
and posterior border doublure very short (sag., exsag.). 
Librigena with prominent, closely spaced terrace lines on antero- 
lateral aspect; coarser lines matching those on cranidium only 
expressed on anterior part of field; posterolateral librigenal corner 
broadly lobate, ventrolateral margin bowed in, single prominent 
terrace line forming sharp posterolateral rim; field with moderate 
dorsal convexity, sculpture excluding anterior terraced lines smooth; 
posterior margin with strong posterior convexity; eye relatively 
large, exsagittal length slightly more than twice the width (tr.), 
doublure broad and robust; vincular furrow strong posteriorly be- 
neath posterolateral corner, becoming effaced anteriorly. 
Rostral plate subtrapezoidal; length (sag.) 40% of width; anterior 
margin with gentle, even anterior convexity; posterior margin nearly 
transverse, with slight sagittal posterior bulge; connective sutures 
obliquely inclined at about 45 degrees, laterally concave; ventral 
aspect with prominent, coarse, terraced lines, larger and more 
widely spaced (sag., exsag.) anteriorly; reentrant dorsal flange 
incompletely known, but robust. 
Hypostome with sagittal length about 80% of maximum width 
across anterior wings; anterior margin (hypostomal suture) with 
lobate ‘M’ shape; anterior wings broad and nearly spatulate; wings 
grading into middle body posteriorly, but separated anteriorly by 
trough-like furrow delineating anterior part of body; lateral border 
narrow, sharply defined and ridge-like; lateral border furrow narrow 
and deeply incised; middle furrow deep, deepest laterally, fully 
impressed medially, with strong posterior curvature; middle body 
moderately inflated, lacking sculpture; maculae small but promi- 
nent, set just behind middle furrow; lateral border furrow grading 
without interruption into posterior border furrow; lateral border 
grading without interruption into posterior border; posterior border 
with posterior convexity nearly identical to that of middle furrow. 
Thoracic segments poorly known; articulating half ring and ring 
furrows not discernible; axial furrow defined only as break in slope 
from pleura; axis with broad transverse convexity; prominent ful- 
crum on pleural lobe, 70-80% distance abaxially. 
Pygidium with length (sag.) 55-60% of maximum width; axis 
with anterior width just under 40% of pygidial width; anterior 
margin transversely straight between fulcra; fulcrum set at 75% of 
distance between sagittal plane and lateral margin; prominent, 
subtriangular articulating facet forming obliquely inclined, 
anterolaterally directed plane distal to fulcrum; posterior margin 
nearly semicircular in plan view, subelliptical in posterodorsal view; 
pygidium with sagittal profile nearly flat anteriorly, prominently 
vaulted posteriorly, set at nearly 90 degrees to anterior part posteriorly 
near margin; plane of margin declined about 10 degrees from that of 
PLATE 6 
J.M. ADRAIN AND R.A. FORTEY 
anterior flat part of sagittal profile; axis only slightly raised from 
pleura, becoming increasingly less differentiated posteriorly; 
doublure broad, extended forward to rear of axis (about 60% of 
sagittal length from front of pygidium); doublure notched medially 
around termination of axis, protruding forward on either side of axis, 
then evenly arcuate distally. 
DIscuUSSION. //laenus weaveri belongs to Jaanusson’s (1957: 110) 
I. sarsi group, which includes the taxa/. consimilis Billings, 1865, I. 
fraternus Billings, 1865 (see Whittington 1965 for both), J. 
auriculatus Ross, 1967, and I. oscitatus Fortey, 1980 (see also 
Nielsen 1995). The group is characterized particularly by the form of 
the pygidial doublure, with its median notch flanked by anterior 
projections (e.g., Pl. 6, fig. 12), and all species show to greater or 
lesser extent the development of posterolateral ‘flanges’ on the 
librigenae. 
Illaenus weaveri is perhaps most similar to J. auriculatus, from 
the basal Whiterockian (Zone L) of the Antelope Valley Limestone, 
Pyramid Peak, California. However, the species differ in that /. 
weaveri has a relatively longer pygidium, much less pronounced 
medial pygidial doublural notching, a rostral plate that is much 
longer medially, and librigena with a less pronounced lateral flange. 
The vincular furrow of J. weaveri is restricted to the posterior part of 
the librigenal doublure (Pl. 6, figs 3b, 4b), whereas that of J. 
auriculatus is continued anteriorly (Ross 1967: pl. 5, fig. 29). 
Illaenus weaveri differs from I. oscitatus, from the Whiterockian 
of Spitsbergen, in the lack of that species’ prominently pitted 
sculpture and fully defined cranidial anterior border. Additionally, 
the cranidial sagittal profile of J. oscitatus is much more evenly 
convex than that of /. weaveri (compare Fortey 1980: pl. 10, figs 2, 
6, with Pl. 5, figs 3c, 4c, 5c). 
Both J. consimilis and I. fraternus, from the Whiterockian of 
Newfoundland, are distinguished from /. weaveri in the possession 
of prominent terrace lines over the entirety of their dorsal surface, 
including medially on the cranidium, on the librigenal field, and on 
all of the pygidial axial and pleural region. The size and shape of the 
librigenal flange of J. fraternus, however, is similar to that of /. 
weaveri (e.g., Whittington 1965: pl. 45, fig. 17). 
Family CHETRURIDAE Hawle & Corda, 1847 
Subfamily uncertain 
DISCUSSION. Cheirurid subfamilial classification is in a state of 
flux. Several subfamilies (including Cheirurinae, Acanthoparyphinae, 
Heliomerinae, and Deiphoninae) are undoubtedly monophyletic. 
Others (e.g., Eccoptochilinae, Sphaerexochinae, Cyrtometopinae, 
Areiinae) are more problematic, and their status as natural groups 
has yet to be convincingly established. Two of the genera dealt with 
herein (Kawina Barton, and Mayopyge gen. nov.) would be as- 
signed, by current convention, to the subfamily Sphaerexochinae. 
This taxon, however, is particularly problematic because 
Sphaerexochus itself is a highly autapomorphic genus, the sister 
Figs 1-12 I/laenus weaveri Reed in Gardiner & Reynolds, 1909 1, It. 26003, right librigena, external view, x7.5. 2, It. 26004, left librigena, external view, 
x5. 3a-b, It. 26005, right librigena, external and internal views, x10. 4a-b, It. 26006, left librigena, external and internal views, x7.5. 5, It. 26007, 
thoracic segment, dorsal view, xS. 6, It. 26008, thoracic segment, dorsal view, x5. 7a-c, It. 26009, pygidium, dorsal, posterior, and left lateral views, 
x3.5. 8, It. 26010, pygidium, dorsal view, x5. 9a-b, It. 26011, right librigena, ventrolateral and external views, x10. 10a-b, It. 26012, pygidium, dorsal 
and posterior views, x7.5. 1la-b, It. 26013, pygidium, dorsal and posterior views, x5. 12, It. 26014, pygidium, ventral view, x10. 
Figs 13-16 Kawina divergens (Reed, 1945). 13, It. 26015, left librigena, external view, x6.5. 14, It. 26016, hypostome, ventral view, x10. 15, It. 26017, 
cranidium, oblique view, x10. 16, It. 26018, cranidium, dorsal view, x10. 
