140 SILURIAN TRILOBITES. 



Originally identified by its discoverer Portlock with the little known Asaphus dilatatus 

 of Dalman. But Dalman's figure is far from good (that by Sars, in Oken's Isis, shows 

 clearly that it is not even of the same genus), and Portlock's specimen was incomplete. 

 Captain, (now Sir Henry) James afterwards obtained a noble series, which were figured in 

 Decade 2 of the Geol. Survey. They are all from one locality : viz. — 



Locality. — Llandeilo Flags. Schists of Newtown Head, Waterford (Mus. Pract. 

 Geology). Museum of Irish Industry. I do not know tliat this fine species exists in other 

 collections, except that of Major Austin, Clifton. 



Barrandia (Homalopteon) radians, M'Coy. PI. XIX, figs. 1 — 4. 



Ogygia Portlockii, jun., Salter. Decades Geol. Surv., No. 2, pi. vii, figs. 3, 5, 1349. 



— RADIANS, M'Coy. Annals Nat. Hist., 2nd ser., vol. iv, p. 408, 1849. 



— — Id. Synopsis Pal. foss. Woodw. Mus., p. 149, pi. i r, fig. 2,1851. 

 Bakrandia, fip. Id. 1. c, p. 149. 



B. (HoMAL.) minor, 1^ unciam longa, late ovalis. Glabella genis angustior, antice 

 hand gihha, illobata vel lohis obscuris. Tliorax axe lavi, pleuris huge sulcatis. Cauda 

 axe conico 4 — 'b-annulato, ajnce rotundafo : axe bis laiiore limbo fere, utrdque sulcis tribus 

 curvis interline atis. Sulci lo7igi, primmn recti, dein profundiores, abrupte recurvi. 



My friend Prof. M'Coy, while animadverting, in the excellent work above quoted, 

 (p. 149), on my mistake in placing this as the young of B. {Ogygia) Portlockii, apparently 

 did not perceive that he also was referring to Barrandia the species he had in the same page 

 described as an Ogygia. The truth is, my mistake and his own arose very naturally from 

 the paucity of specimens. And as Burmeister had six years before predicted the occur- 

 rence of the metamorphosis among Trilobites, I was not so far out as my friend and critic 

 supposed. With few exceptions, and those not very clearly made out, or even founded 

 on mistake, — the increase of the number of segments with advancing age was not known 

 till 1849, when M. de Barrande published his beautiful series of observations on the geniu 

 Sao ; and at the same time, without knowing of his work, my own figures of this species 

 were given. In 1851 M. Barrande examined the British specimens, and satisfied himself of 

 the true occurrence of the metamorphosis ; and that in a genus to which he had not access 

 in Bohemia. Barrandia and Ogygia are northern types. 



As I am not sure that I shall be able to show this change of form with age in any 

 other British species, I would refer particularly to it here. 



M. Barrande, to w^hora the real credit must belong of working out intentionally a point 

 only accidentally observed by others, has established the fact that Trilobites — probably all of 

 them — undergo a distinct and regular series of changes in the young state, comparable 

 very nearly to that observed in the other Crustacea. The developuient of the eyes and 



