some British Fossil Crustacea. 409 



Not uncommon in the black Wenlock shale of Pen Cerrig, 

 Builth. 



{Col. University of Cambridge.) 



Barrandia (M^Coy), n. g. 



Gen. Char. Body ovate, depressed ; ce'phalic shield semicii'cular, 

 with the lateral angles produced backwards into 

 short spines; glabella widely clavate, the axal 

 furrow strong and nearly parallel at the base, 

 becoming obsolete towards the front ; eyes large, 

 narrow, reniform ; eye-line behind the eye cut- 

 ting the posterior margin about the middle, in 

 front of the eyes arching forwards, first out- 

 wards and then inwards ; thorax of seven seg- 

 ments ; aocis convex, nearly as wide as the pleurse, Barraiidia. 

 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 

 dift'ers 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 segmentalfurrows 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*. 



Barrandia Cordai (M'Coy). 

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



* Since the above was written Mr. Salter has figured (2nd Decad. Geol. 

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

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

 species of Ogygia (0. dilatata, Phil., 0. PortlocH, Salt.). My reasons for 

 dissenting from this view 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 Ogy- 

 gia Bttciii, 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 Builth, 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. 



