3G SILURIAN TRILOBITES. 



and which has both the apex of the tail and the front of the head more prominent and 

 produced than in the other variety. The North Welsh specimens are intermediate, the 

 cheeks steeply bent down, the margin of the eye-lobe narrower, and the lenses 

 more numerous ; occasionally ten in a row, in the widest part of the eye. 



P. macropJdhalma of Brongniart, with which the first describer of this species united 

 it, has neither the obsolete basal lobes, nor greatly expanded glabella, nor the large 

 eye of the one we have been describing. And there can now be no question of uniting 

 either of these with the Dudley fossil included by Brongniart (described above as P. 

 S'o/ccsii), all subsequent writers having adopted the distinctions. 



It is, however, worth Avhilc to compare our species with the fossil figured by Dr. 

 Pander, in his ' Beitriige zur Geogn. Russ.,' pi. v, fig. 7, and pi. vi, fig. 9. It has large 

 eyes like ours, but the glabella-lobes differ considerably. And at all events, it is not the 

 P. maeroptlrflialma, to which he refers it. 



Localities. — Caradoc, Tyrone, abundant ; Kildare ; Wexford ; and AYaterford, in 

 Ireland. Llanfyllin and Llanwddyn, Montgomeryshire, in North Wales. (Mus. Pract. 

 Geoloo-v, &c.) 



Sub-<jenus — Ciiasmops, M' Coy. 



Though not quite convinced that this is a definable sub-genus or section of Phacops, 1 

 adopt it because it conveniently bridges over the space between Acaste proper and the 

 typical species of Odontochile. The P. co)wp//f/ia7mus,Boeck, is the type of the section, and 

 has to perfection the enlarged upper lobes of the glabella and the expanded forehead- 

 lobe, while the tail is only of moderate size, and has but eight or nine segments. 



P. macroura, again, is typical so far as the head is concerned, but has very numerous 

 segments to the tail, a characteristic still further carried out in such species as P. trim- 

 cato-caudatus, which may be considered as linking on the sub-genus C/tasmops to Odon- 

 tochile, as P. Jukesii links it to the ordinary Acaste. 



Altogether the group has a wide range, and tends to show the unity of the genus 

 Phacops. 



Piiacops (CiiAsMors?) Jukesii. PI. I, figs. 29, 30. 



1 Piiacops sclerops, Batman. Palaeadae, t. ii, fig. 1, y, icon mala, 1826. 

 Piiacops Jukesit, Salter. Decides Geol. Surv., No. 7, Art. 1, p. 11, August, 1S.">3. 

 — — Id., in Morris Catal., 2nd ed., 1854. 



