OGYGIA. 131 



urceolata, antice clavata, lobis obsciiris. OcuU modici, circumscrijoti. Thorax articuUs 

 rectis, fulcro remoto, sulco pleurali profundo, apicibus obtusis. Cauda axe primum 

 conico, dein parallelo, 7-8-cosfafo, lateribus 8-cosiatis, costis rotundis rectis simplicibus, 

 ad marginem striatum concavuw, undulatis. 



The cabinet of Mr. Griffith Davies, of Islington, has for several years contained a fine 

 series, from Builth, of this Trilobite, which has been so long obscure, from the very 

 imperfect specimen figured in the ' Silurian System.' Mr. James Sowerby's figure showed, 

 indeed, that it was distinct from the Ogi/gia Buchii ; but its characters could not be 

 fully known from the fragment figured. The exact age of the Cornclon mountain, 

 relatively to the trappean beds of Builth, may now probably be determined from this 

 single fossil. 



It is a large species ; occasionally 4 1 inches by 3|, depressed, but not flat; of an obtuse 

 oval shape, with rather blunt extremities in the ? form (figs. 9, 11), or with the head 

 somewhat pointed in the o varieties (figs. 1, 7). I will describe the broader 'form first. 



Head a broad segment of a circle, the length being to the breadth nearly as 2 to 5. 

 Of this breadth the glabella occupies more than a third and less than a fourth ; it is 

 urceolate in shape, being broad below, contracted at the sides rather below the eye, and then 

 again clavate above. Traces of two short furrows only on each side (the three upper 

 ones are obsolete in the adult), the lower one obhque ; — it is the neck-furrow, incomplete 

 in the middle, but marking off a much broader segment beneath the glabella than below 

 the cheeks, where the furrow is strong and continuous. 



In front the blunt glabella invades the striated margin, which is here narrow, but very 

 broad round the cheeks, and produced behind into an abruptly narrowed spine, which 

 reaches the sixth thorax-segment in the adult ; it is shorter in the young. The eye is 

 rather small for the genus, placed fully half-way up the head, near the glabella, and over- 

 hung by it ; a depressed space surrounds it. The facial suture cuts the posterior margin 

 rather more than half-way out, and in front it circles round the glabella (intra-marginal). 

 The labrnm (fig. 10) is pointed, with two pairs of furrows, and a broad arched base. It 

 is also visible in fig. 9. 



The glabella-furrows are strongest in young specimens, and get partly obliterated in 

 age. In some young ones four pairs of furrows show clearly; the two upper ones directed 

 obliquely backwards towards the eye, the middle one obliquely forwards, the basal one (neck- 

 furrow) direct across. Besides these there is the narrow marginal furrow frequent in 

 the genus, and which probably marks out the articular area on the under surface of the 

 crust. All the glabella-furrows are short and shallow, except the hindermost pair, which 

 are deeper, and show in every specimen of all ages. 



Thorax with a wide axis, less than one third the whole width, the rings straight 

 across, and ornamented with coarse arched strise. The pleurae straight, scarcely bent at 

 the remote fulcrum, very deeply grooved and strongly facetted ; the apices blunt and 

 rather tumid. The fulcrum is about half-way out in all but the front rings. 



