II. 



III. 



Ll-0 mm. 



1 0-0 mm. 



L9-0 „ 



15-0 „ 



6-0 „ 



5-0 „ 



7*5 „ 



o-o „ 



64 TRILOBITES OF GIRVAN. 



thirds or three fourths the width of the axis from the axial furrow. The axis is 

 conical with rounded end, independently convex, rising above the general surface 

 of the pygidium, and circumscribed by broad shallow axial furrows, especially well 

 seen in casts ; its width is about one third that of the pygidium, and its length 

 rather less than half that of the pygidium. A straight median groove is seen 

 in some casts behind the axis. The fascia is also seen in casts to extend from the 

 posterior margin to the tip of the axis, thus being about half the length of the 

 pygidium. The lateral lobes are flattened, lower than the axis, with the edges 

 steeply bent down, and there is no furrow behind the fulcrum. 

 Dimensions. — 



I. 



Length of pygidium . . 17"0 mm. 



Width of „ . . 23-0 „ 



Length of axis . . 8 - 5 „ 



Width of „ . . 10-0 „ 



Affinities. — The proportions of the head-shield are different from those of 

 T. boivmani, and M'Coy (op. cit.) has pointed out its distinctive features. The 

 pygidium differs by the possession of the prominent well-marked axis, more 

 strongly truncated lateral angles and absence of post-fulcral furrow. 



From I. portlocJd, Salter, it differs by the narrower more prominent axis, and 

 rather more remote fulcrum ; but in the smaller specimens of /. portlocki figured 

 by Salter 1 this last point of difference is not so apparent. The groove behind the 

 axis and the truncation of the lateral angles are points of agreement. 



In Salter's figure of the pygidium ascribed to I. crassicauda, from Bugon, the 

 axis does not appear so well defined, but this is mainly due to the state of 

 preservation of the specimen ; and in other respects (width of axis, position of 

 fulcrum, truncation of angles, caudal fascia) this specimen is identical with the 

 Minuntion form. 



Of American forms, I. americanus, Billings, 2 appears to be very closely 

 allied to /. latus ; the head-shield, however, seems rather more convex, and the 

 lateral angles of the pygidium less abruptly truncated. In the strongly marked 

 and convex axis, and the degree of truncation of the lateral angles of the 

 pygidium, I. latus resembles I. (Thaleops) ovatus, Conrad, 3 from the Trenton 

 Limestone. 



Collection*. — Mrs. Gray; Museum of Practical Geology. 



Horizons and Localities. — Stinchar Limestone Group (Llandeilo) : Minuntion, 

 ? Balclatchie Group (Llandeilo) ; Ardmillan. Balclatchie Conglomerate. 



1 Salter, ' Mon. Brit. Trilob.,' pi. xxvi., fig. 4. 



2 Billings, 'Canad. Nat. and G-eol.,' vol. iv, 1859, p. 371 ; Clarke, ' Lower Silur. Trilob. Minnesota,' 

 1894, p. 714, figs. 20—23. 



3 Conrad, 'Proc, Acad, Nat. Sci, Pliilad.,' vol. i, 1843, p. 332 ; Clarke, op. cit., p. 71G, figs. 25—27. 



