BRITISH FOSSILS. 



impression of part of it on this exposed side — but there is none. And 

 the probability that we have in this case the whole number of rings 

 is strengthened by observing what takes place in one a little older, 

 fig. 5*, for in this the last segment is very narrow, and appears to be 

 in course of development.* We have no perfect very young spe- 

 cimens of O. Buchii, and therefore the point is still open to observation 

 in this genus, in which occasional specimens have certainly been found 

 with seven rings, as noticed by Quenstedt, Dalman, and Burmeister. 

 The usual number is eight. 



Affinities. — Its identification with O. dilatata by its discoverer, Port- 

 lock,! is easily understood by reference to his very imperfect original 

 specimen, which he saw clearly was distinct from O. Buchii. With 

 Dalman's figure, too, there is more similarity than with the better 

 figures published since by Sars (Oken's " Isis/' 1835), and Emmerich 

 (" Scientific Memoirs,*' vol. iv.) Our species differs from the Swedish 

 one essentially in the broad glabella, narrow head margin, and forward 

 eyes. The proportions of the caudal axis and the lateral ribs of the 

 tail are altogether different, but the above-mentioned characters of the 

 head will suffice. 



British Localities and Geological Range. — Llandeilo flags. Large 

 and fine specimens were obtained in abundance at Newtown Head, 

 Waterford ; smaller ones are common at Builth, Radnorshire. 



Foreign Distribution. — Not known. Dalman's figure of A. dilatatus 

 is more like our species than the representations given by Sars and 

 Emmerich, but we take it for granted these authors figure Brunnich's 

 species, as their specimens were from Christiania. 



Explanation of Plate VII. 



Fig. 1. Large, but not full-grown individual. Newtown Head, Waterford. 



Fig. 2. Same, or similar specimen, dissected. 



[The letters on the head are placed in the order of those in the Mem. Geol. 

 Survey, vol. ii., pt. 1, p. 334,] 

 a, Forehead, or front lobe ; h, neck lobe, traversed by a pair of oblique lines c c, 

 which mark the extent of the articulating surface of the first thorax joint ; g, 

 basal, or first lobe; h, middle, or second lobe; i, upper lobe, third; j, posterior 

 margin, or lateral neck lobe ; C, free cheek, or wing ; o, margin. 



Fig. 2*. Thorax ring of ditto; cc, place of fulcrum; e, an oblique line running up 

 wards to the furrow (as in O. Buchii). 



* We believe this increase in num.ber is not a solitary case. The figured specimen of 

 Trinucleus Lloydii, Murchison, has but five rings, and seems perfect. It may be peculiar 

 to certain genera. Burmeister has suggested it in the case of Paradoxides bohemicus. 



t We have great pleasure in dedicating the new species to Lieut.- Col. Portlock, whose 

 careful and elaborate treatise on the Irish trilobites will be always valued by the British 

 student. It was not only the best, but, at its publication, was the only English work 

 containing extended views of the generic affinities. 



