136 SILURIAN TRILOBITES. 



narrower axis, and with less distinct interlineations to the sides. From Barrandia 

 Portlockii (see PI. XIXj, the much greater length of the axis, and narrower glabella, wiU 

 easily distinguish it. 



Locality. — " Arenig group " of dark earthy slates at Whitesand Bay, St. David's 

 Pembrokeshire, north of the reef called Trwyn hwrddyn, which is probably of the age 

 of the Tremadoc Rocks. 



Ogygia Selwynii, Salter. PI. XVII, figs. I — 7. 



AsAPHUS Selwynii, Salter. Reports Brit. Assoc. Trans, of sect., p. 57, 1852. 



— — Id. In Morris's Catal., 2nd. ed., p. 100, 1854. 



Ogygia Sklwynii, Id. Append. Ramsay, Geol. N. Wales ; Mem. Geol. Survey, 

 vol. iii, 313, pi. ix, figs. 2—6, and pi. 11 b, fig. 5, 1866. 



0. elliptica plana, triuncialis, capite brevispinoso ; glabella oblongd, gents angustiori. 

 Sulci glabellares brevissimi. Oculi magni. Thorax pleuris a fulcra remoto decurvis. 

 Cauda axe angusto, longiconico, 6, 1-sulcato, apice prominulo ; lateribus 1-sulcatis, sulcis 

 brevibus, obscuris, interline atis. Fascia latissima, striis confertis. 



A species of more than ordinary interest, since it was the first true indication of the 

 fauna of the great group now distinguished in Britain under the name of the " Areiiig or 

 Skiddaw group." A solitary fossil, found by Prof. Sedgwick and myself in 1844, proved 

 different from all other Lower Silurian species, and characterised a particular set of strata, 

 which were successfully compared with the rocks west of the Stiper Stones, by means of a 

 specimen of this same species. Such exact indices are our friends the Trilobites of age 

 in the Palaeozoic Rocks.^ We have specimens from three localities, viz., HengwTt Uchaf, 

 four miles north-east of Dolgelly, and we figure the original small 

 Fig. 31. specimen (Pig. 31); from S. Carnarvonshire; and lastly from near 

 Chirbury, Shropshire. The latter are by far the finest specimens, and 

 our larger figures are taken from them. Mr. Lightbody has lent some 

 Ogygia Selwynii. fragments of the head, and our restored figure (PL XVII, fig. 1 ) is 

 from 001^611™^" partly made from these, and partly from others in the Museum of Prac- 

 tical Geology. 

 The head is broad, semi-oval, an inch and a half broad or more in Shropshire specimens, 

 and broadly margined. The oblong glabella is distinct all round, depressed, gently 

 clavate in front, and contracted at the sides, but otherwise parallel-sided ; with a large 

 upper lobe and three very short lateral lobes, which do not reach even one third across 

 the glabella (fig. 2), the lower pair largest and somewhat oblique. Eyes large lunate, 

 placed less than half-way up the head. The facial suture cuts the margin more than 



^ Tlie " Skiddaw group " lias since proved rich in Graptolites, and these enable us to compare it 

 with the Quebec group of America and the black slates of Melbourne. But Oyyyia Selwynii was the first 

 step in the comparison for the whole series. 



