68 TRILOBITES OF GIRVAN. 



Affinity's. — The broad shape of the pygidium, strong truncation of the lateral 

 angles, absence of post-fulcral furrow, approximate fulcrum, and wide axis mark 

 it off from I. bowmani. 



From I. lata* it differs in the broader less defined axis, more approximate 

 fulcrum and less strongly truncate lateral angles; but they seem rather closely 

 allied. 



I. americanus, Billings, 1 from the Trenton Limestone, appears to belong to the 

 same group and seems more closely allied to I. portlocki than to I. latus. 



Collections. — Mrs. Gray; Museum of Practical Geology. 



Horizon and Locality. — Stinchar Limestone Group (Llandeilo) : Craighead. 



12. Illgenus shallochensis, sp. no v. Plate X, figs. 2—5. 



Specific Characters. — Head-shield semi-elliptical, rounded, appearing from 

 above to be nearly as long as wide owing to the sides being bent down nearly 

 at right angles. Anterior portion in front of glabella strongly bent down with 

 convex surface. Posterior margin of head-shield with slightly concave outline. 

 Glabella with gentle independent convexity, occupying nearly the middle third 

 of head-shield and extending less than half its length. Narrow depressed occipital 

 band seen in cast. Axial furrows well marked and parallel, but curving outwards 

 slightly at anterior end. Fixed cheeks with base a little over half as wide as the 

 glabella; palpebral lobe slightly elevated, of moderate size, about its own length 

 from posterior margin, and situated a little behind the middle of the glabella. 

 Shallow broad groove extending across base of fixed cheek and on to free cheek 

 around eye. Facial sutures with posterior branch straight and vertical to posterior 

 margin of head-shield ; anterior branches nearly parallel but curving inwards near 

 front margin to meet it at about 45°. Free cheeks narrow, nearly twice as long as 

 wide, but wider than fixed cheek, pointed in front ; genal angle well rounded, not 

 quadrate. Eyes of moderate size, with broad shallow groove round base continued 

 behind across base of fixed cheek. Surface of head-shield ornamented with coarse 

 circular pits scattered irregularly amongst numerous fine punctge. Free cheek 

 continued forwards beneath the sharp lateral margin of the head-shield as a 

 flattened inferior marginal band ornamented with longitudinal parallel ruga3 con- 

 tinuous with those on epistome. Epistome bounded and truncated by the epistomial 

 sutures meeting the front edge of the head-shield at about 75°. Anterior and 

 posterior margins of epistome with almost similar curvature in opposite directions. 

 Epistome measures from back to front about half as much as from side to side 

 on front edge ; and 8 — 12 coarse rugse traverse it from side to side. 



1 Billings, 'Canad. Natur. and Geol.,' vol. iv (1859), p. 371; J. Clarke, "Lower Silur. Trilob. 

 Minn. " ; ' Pinal Rep. Geol. Nat. Hist, Surv. Minn.,' vol. iii, pt. 2 (1894), p. 714, figs. 20—23. 



