4 



BRITISH FOSSILS. 



must have figured two species under the name. We cannot therefore identify the Irish 

 fossil with his, especially as the decided bend in the pleurae is not given in his figure ; 

 and we dedicate the species to Col. Portlock, who has so well elucidated the trilobites of 

 Ireland. Our description is drawn from his specimen. 

 Locality. — Lower Silurian rocks of Tyrone, Ireland. 



4. I.perovalis, Murchison, Sil. Syst. (1839) t. 23, f. 7. 



/. capite gibboso, oculis remotis ; thorace quam caudd longiore, segmentis 10, axi angusto ; 

 pleuris paulo re/lexis ad fulcrum, quod pleurarum fere omnium ad tertium positmn est, cauda 

 axi angustissimo, circumscripta. 



In the young specimen figured the eyes are not seen, but they must be nearly as dis- 

 tant from the axal furrow as the width of the central lobe ; the axis of the thorax is much 

 narrower than the pleurae — this is maintained in the adult; the fulcrum at about one- third 

 in the first segment, in the next a little farther out, and from thence at an equal distance 

 in all the segments. Tail in youth more than a semi-circle, shorter than the thorax, with 

 the axis very narrow, circumscribed, and reaching about half-way down ; in the older 

 state it forms about a semi-circle, is flattened, and the axis does not reach so far. The 

 end of the axis does not indent the broadish incurved margin of the inferior side ; it does 

 so in /. crassicauda, which has a shorter and subtruncate convex tail, a much broader 

 axis to the thorax, and the eyes not nearly so remote. We have not specimens perfect 

 enough to characterize /. perovalis fully, but enough to show that it ought not at present 

 to be connected with the Swedish species. 



Locality. — Llandeilo flags of the Corndon Mountain, Shropshire. 



5. /. ocularis, — n. sp. 



I. capite semicirculari, idraque acutangulo, loho centrali angusto; oculis magnis, approxi- 

 matis, a cervice vix dimidium diametri remotis: thoracis axi pleuris aquali, his ad fulcrum 

 defexis et paidum refexis ; pygidio ? 



A species so remarkable among the trilobed division of Illa-nus, for the size of the eye, 

 that we have ventured to name it — we cannot recognize it among published species. 



Locality. — Llandeilo flags, county Kildare, Ireland. 



6. /. Murchisoni, Salter. /. Mosenbergii, ib., Mem. Geol. Surv., vol. ii., pt. 1, pi. 5, f. 6-8. 



/. capite cauddque magnis, thorace brevi, anterius contracto ; capite iitrdque acidangulo, 

 oculis ad sulcos approximatis, diametrum suiim a cervice remotis ; thorace axi latissimo dis- 

 tincto, pleuris anterioribus brevissimis. 



A species connecting the true Illceni with Bumastus, having a very wide axis and very 

 narrow front pleurae. The axis, however, is really distinct, and the fulcrum not close to 

 it. There is reason to believe that the /. Eosenbergii, to which these specimens were 

 referred before, is a very large specimen of /. crassicauda : as Pander and Burmeister 

 have previously stated. This fine Illanus, from near the classic town of Llandeilo, we 

 dedicate to Sir R. I. Murchison. 



Locality. — Llandeilo flags of Llandeilo, South Wales. 



Notwithstanding its great similarity in external characters to Nileus, Illcenus is essen- 

 tially distinguished by the possession of a rostral shield, a portion so designated by Bur- 

 meister, and peculiar to Illcenus and Calymene. This shield appears to be a plate inserted 

 between the two cheek-pieces, and separates the hypostome, or labrum, from the facial 

 suture. In Asaphus the cheek-pieces are united across, or in some divided merely by a 

 vertical suture. III. centrotus (Dalman) has long head-spines, and in this differs from most 

 species of the genus, though some others are angular at the corners. Pander, and after 

 him Sars, described the hypostome ; it is simply oval, and by no means notched, as in 

 Asaphus. 



The genus commenced in the Llandeilo flags, and continued to the end of the Silurian 

 system. We think there is not good evidence of its longer duration, though Munster cites 

 one or two species from Devonian rocks in Germany. 



June, 1849. 



J. W. Salter. 



