OLENUS TRUNCATUS. 53 



The British specimens of Oleum truncatus with which I am acquainted are 

 invariably very badly preserved and have often been mistaken for Olenus cataractes. 

 Belt, however, recognised the presence of the species in the Dolgelly district. From 

 0. cataractes it is easily distinguished by the shortness of the glabella, which is only 

 about half as long as the head, while in 0. cataractes it extends nearly to the anterior 

 margin. The breadth of the cheeks, the distance of the eye from the glabella, and 

 the triangular tail with five segments to the axis also serve to distinguish 0. truncatus 

 from 0. cataractes. 



In the very imperfect state in which the specimens are usually found in Britain 

 it is not always easy to separate 0. truncatus from 0. gibbosus. The head of Olenus 

 truncatus is broader in proportion to its length and is straight or even emarginate 

 in front, while in 0. gibbosus the outline is nearly a segment of a circle. The genal 

 spines of 0. truncatus are directed somewhat outwards, while in 0. gibbosus they 

 run directly backwards. The axis of the thorax is narrower in 0. gibbosus, and 

 there are fifteen instead of thirteen thoracic segments. In Olenus truncatus the 

 axis of the tail is broad and consists of five segments, and the lateral lobes are 

 narrow ; in 0. gibbosus the axis is narrow and consists of at least seven segments, 

 while the lateral lobes are comparatively broad. The margin of the tail is entire 

 in Olenus truncatus, while in 0. gibbosus it bears a small spine on each side, but it 

 is not often that the spine is sufficiently well preserved to be distinct. 



Olenus micrurus is easily distinguished by its long glabella and small quadrate 

 tail; 0. longis^pinus by its long backwardly-directed genal spines and its broad and 

 short tail, which has a circular rather than triangular outline. 



Synonymy. — The earlier writers do not distinguish between the Triloba* 

 truncatus of Bri'mnich and the Entomostracites gibbosus of Wahlenberg, and 

 Briinnich's own account would apply equally well to either species. He gives the 

 number of segments as twenty, but probably he included the rings of the tail, and 

 it may be remarked that he reckons twenty-four segments in his Trilobus 

 [Calyme7ie~] tuberculatus. Wahlenberg identifies Briinnich's species with his own 

 Entomostracites paradoxissimns, which is a Paradoxides. Dalman and Burmeister 

 give Trilobus truncatus and Entomostracites gibbosus as synonyms. 



Angelin appears to have been the first to recognise the differences, and his 

 definition of the two species has been accepted by most subsequent authors. 



The form Trilobites gibbosus var. of Boeck and Olenus gibbosus var. of Kjerulf is 

 not sufficiently described by those authors to be identified, and it is on the 

 authority of Brogger that I have included it in the synonymy. The species 

 figured by Brogger himself and referred by him with some doubt to Olenus 

 truncatus shows six distinct segments in the axis of the tail. 



Horizon and Localities. — Lower Lingula Flags: Dolgelly; Cae Gwernog, 

 Mawddach Valley. 



