148 CRUSTACEA. 



tapering towards the pygidium ; 'pleura flat, their ends slightly- 

 falcate and bent backwards, no facets, a slightly oblique sub- 

 mesial pleural furrow not quite reaching the end ; pygidium 

 semicircular, entire, having very few simple segmental furrows 

 placed near the anterior margin (one to three in number) ; 

 axis short, having one to three small segmental furrows. 



This I conceive to be a subgenus of Ogygia, from which it 

 differs in its fewer thoracic segments, and having but very few 

 and simple ribs to the tail. The genus agrees with the descrip- 

 tion given by Hawle and Corda of their genus Alceste, with the 

 exception of this having seven thoracic rings and that having but 

 four ; it is remarkable that Alceste is figured by those authors 

 with three segmental furrows to the pygidium, while this has only 

 one, making the total number of segments visible the same in 

 both ; the number of the pygidial segments is however of course 

 liable to vary with the species, while the thoracic ones are sup- 

 posed to be constant. I know but one species, the following*. 



Bairandia Cordai (M'Coy). 



Sp. Char. Length one-fourth more than the width, length of 

 head, thorax and pygidium almost equal ; cephalic shield 

 slightly more than twice as wide as long, lateral angles very 

 short ; eyes half their length from the axal furrow ; pygidium 

 depressed, length rather more than half the width, axis two- 

 thirds the length, conical, segmental furrows one on each side, 

 obtuse. Length 11 lines. 

 Black Wenlock shale of Builth. 

 (Col. University of Cambridge.) 



Ampyx latus (M'Coy). 



Sp. Char. Entire animal transversely ovate, length one-fifth less 

 than the width ; cephalic shield smooth, front margin regularly 

 curved, width three-fifths the length ; glabella moderately tu- 

 mid, pyriform, having a narrow vertically elongate (? ocular) 

 swelling close to the middle third of each side, and two short, 



* Since the above was wi-itten Mr. Salter has figured (2nd Decad. Geol. 

 Surv. pi. 7. f. 4) a species of this genus, with thi-ee segments to the pygi- 

 dium, which he gives without any apparent reason as the young of an Irish 

 species of Oyygia (0. d'datata, Phil., 0. Portlocki, Salt.). My reasons for 

 dissenting from this vievv are, 1st, it is contrary to analogy of other allied 

 Trilobites to suppose that the young and adult differ in the number of their 

 thoracic segments; 2nd, in the Cambridge collection, specimens of the Oijy- 

 gia Buchi, half an inch wide, have exactly the same number of segments 

 and other characters as an adult six inches long ; 3rd, the supposed young 

 has only been found at Buihh, where the Irish species, his supposed adult 

 thereof, has never been found, being only known in the schists at Waterford, 

 where it abounds, but where the supposed young have not occurred. 



1 



