22 BRITISH CAMBRIAN TRILOBITES. 



Synonymy. — This is one of the forms which was described and figured by 

 Salter as Agnostus princeps, a species which ho divided into two varieties, var. 

 ornatus and var. rudis. The specimens which ho ascribed to the former variety 

 belong to his species A. trisectus, while in var. rudis he included distorted specimens 

 of A. piriformis from the Lower Lingula Flags, and specimens of the present 

 species from the Upper Lingula Flags. As A. pisifomiis and .1. trisectus are 

 already provided for, it might have been possible to restrict the name J. princeps 

 to the form here described. But, unfortunately, by far the greater number of 

 specimens which have been identified as .1. princeps by Salter and others, belong 

 either to .1. piriformis or to A. trisectus; and therefore, to avoid confusion, it seems 

 better to drop that name altogether and to employ the alternative rudis, although 

 this name is by no means descriptive. 



Type. — The original specimens on which Salter's fig. 3 is founded are in the 

 Geological Survey Museum (registered as nos. 872o — 8728). It is, however, 

 impossible to say exactly which of these specimens was used. The figure appears 

 to be composite. 



Horizon and Lucidities. — Upper Lingula Flags: Penmorfa; Ogofddu, near 

 Criccieth, with A. trisectus (collected by Mr. W. G. Fearnsides). 



1 ( .». Agnostus sidenbladhi, Linnarsson. Plate 11, fig. 17. 



1869. Agnostus sidenbladhi, Linnarsson, Oni Vestergotlands Camb. och Sil. AJiagr., p. 82, pi. ii, 



figs 00, 61. 



Head quadrate, with a broad margin, which is widest at the anterior angles 

 but narrows considerably towards the genal angles. Glabella about two thirds 

 the length of the head and one third the width, nearly parallel-sided, rounded in 

 front ; the anterior lobe is very indistinctly marked off from the posterior by a 

 very faint furrow ; the posterior lobe forms about two thirds of the whole length ; 

 basal lobes small. Cheeks nearly equal in width throughout, confluent in front of 

 the glabella. 



Thorax: axis wide, lateral nodules small, and not very clearly marked off 

 from the central portion; pleura? grooved. 



Tail quadrate, with a wide margin, which is produced at the posterior angles 

 into a short point. Axis short, with nearly parallel sides, rounded behind ; anterior 

 segment reduced to two small lobes at the anterior angles ; middle segment also 

 very short, produced forwards between the two lobes which represent the anterior 

 segment, bears a, median keel or tubercle, which is most prominent at its posterior 

 extremity. The third segment Forms about two thirds of the whole axis. Lateral 

 lobes uniform in width throughout, confluent behind the axis. 



