130 SILURIAN TRILOBITES. 



shire. The arrow indicates the direction of the pressure, which has shortened all the 

 parts ; and the wrinkles on the surface are in the mean direction of that force, which 

 was undoubtedly, as Prof. Phillips, Prof. Haughton, Mr. Sharpe, and others have shown, 

 the cause of this common phenomenon in the slaty rocks. Lateral pressure, acting on a 

 confined plastic mass, is quite sufficient to produce such effects ; and has, indeed, been 

 proved by experiment to be equal to the result. Mr. H. C. Sorby has philosophically 

 illustrated this change of dimensions in the rock, which is noted here the more par- 

 ticularly, as our future plates will show several species of trilobites affected by it. 



Ogygia ? (vel Phacops) subduplicata, Salter. PL XV, figs. 7, 8. 



0. Cauda lata semiovatd, suhtrigond, \\ unciam longd, fere pland, costis perdu- 

 plicatis. Axis conicus, per \ cauda extensus, apice prominulo \Z-annulato, annulis 

 anticis ahrupte angulatis, Costa later ales 9, marginem angustissimum fere attingentes, ad 

 apices decurvata, omnino interlineata, sulcis posticis conniventibus, ultimis profundis 

 parallelis nee abhreviatis. 



Although placed in Ogygia, from its general resemblance to that genus, I confess to 

 a strong suspicion that this is a Phacops of the section Chasmops, and as such it should 

 have been previously described. The strong posterior furrows, with their connivent 

 intermediate sulci (or rather it is the intermediate ones which are the longest and deepest), 

 strongly remind us of some Ogygia ; while the character of the axis, with its arched and 

 duplicate anterior rings, is much more like that of Chasmops (see PI. IV, fig. 19). 

 Moreover, it is more likely that this should be a lingering species of the latter subgenus, 

 than that Ogygia should reappear in the Lower Llandovery Rocks, Avhile it is absent 

 from the Caradoc. 



Locality. — Llandovery Rocks of Haverfordwest, Pembrokeshire. The specimens 

 are in the small but excellent collection of Mrs. Breawell, of Cambridge Terrace, 

 Brighton ; relict of H. Day, Esq., formerly of Hadlow, Uckfield. 



Ogygia (Ptychopyge^) corndensis, Murchison. PI. XVI. 



AsAPHUs coRNDEHSis, MurcMson. Silurian System, pi. xxv, fig. 4, 1839. 



— — Salter. Morris's Catalogue, 2nd ed., p. 100, (List at end of 



Asnphus,) omitted in p. 112; 1854. 



— — Id. Siluria, 2nded., pl.iii, fig. 4, 1859. 



0. obtuse ovata, 3 — 4<-uncialis, capite lata semieireulari, spirits parallelis. Glabella 



' PtycJwpi/ge, Angelin, consists of those species of Ogygia which have the facial suture within the 

 margin in front. O. corndensis certainly belongs to it. I do not know sufficiently the structure of other 

 species to subdivide the genus. 



