PHACOPS. 55 



Localities. — [It has been quoted from Lower Silurian rocks, by myself (the Caradoc 

 strata of Robeston Wathen and Narberth, in Pembrokeshire, for instance) ; but I do not 

 know that it really descends below the May Hill Sandstone.] 



May Hill Sandstone (or Upper Llandovery, Murchison), Dingle, Ireland ; Barr, 

 Walsall. Woolhope Limestone and Shale, Malvern ; Presteign ; Woolhope ; Walsall, 

 and from the Denbighshire Grits of N. Wales. Wenlock Limestone and Shale ; the 

 Malvems ; Shropshire ; North and South Wales, everywhere ; Westmoreland ; Dingle, 

 &c. Lower Ludlow and Aymestry Limestone of Shropshire and Herefordshire; 

 of Dudley Tunnel ; of Malvern. Upper Ludlow, Ludlow ; rare in this formation 

 (R. Lightbody). 



The above are localities in general for the common variety. Var. j3 is found in Wen- 

 lock Limestone (Murchison), and our figured specimen from Lower Ludlow, Ledbury 

 tunnel, in Dr. Grindrod's cabinet. Var. y is only known as yet in Lower Ludlow 

 rocks, and specially at Vinnal Hill, Ludlow (Mus. Pract. Geol.) ; also at Newton, in the 

 same neighbourhood (Mr. Edgell's cabinet). Var. 8 is from Dudley. 



Phac. (Odontochile) longicaudatus, Murch. PL III, figs. 19 — 28. 



Trilobite with caudal process, Parkinson. Organic Remains, vol. ii, t. xvii, fig. 



17, 1811. 

 Asaphus longicaudatus, Murchison. Silur. Syst., pi. xiv, figs. 11 — 14, 1837. 



— — Milne Edwards. Crustaces, iii, 308, 1840. 

 Phacops longicaudatus, Emmerich, Gold/uss, fyc. Loc. cit., 1845, 1846. 



— mucronatus, Burmeister. Org. Tril., t. iv, fig. 9 (exclude synonyms), 1843, 



anded. 2 (Ray ed., 1846). 



— caudatus, /3 longicaudatus, Salter. Decades Geol. Survey, ii, pi. i, figs. 



13, 14. 

 Var. a, armiger. PI. Ill, figs. 19 — 21 (P. longicaudatus of authors). 

 Var. /3, Grindrodianus. PI. Ill, figs. 22 — 28, new variety. 



P. (Odontoch.) medius, 2 — 3-pollicaris, ovatus elongatus subconvexus, capite Iato,fronte 

 mucronato. Oculi maximi. Pleura sulcata recurva,fulcro ad dimidium. Cauda trigona 

 longimucronata ; axe 15 — 16-annulato, ad apicem haud eminente ; costis later alibus 7 arcu- 

 atis, duplicatis, ad marginem lavem abrupte terminatis, recurvis. 



Distinguished easily at a glance from the more common P. caudatus by the greatly 

 elongate tail, but much less distinct when closely examined, since the general habit and 

 even the minute structure is similar to that of the Wenlock species. 



In all the specimens I have seen (and there are few perfect ones extant of the long- 

 tailed species) the following differences are observable: — 1st. The axis is invariably 

 narrower, not one third the width of the head, always quite as much so as in the P. 

 caudatus, var. nexilis, which differs from the other varieties in this particular, and yet has 

 less trace of a mucronate front than any of them. 2nd. The abrupt mucro (not mere 

 angulation) to the front. 3rd. The smaller tail, with more arched furrows. 4th. The 



