AGNOSTUS GIRVANENSIS. 5 



Axis broad, subcorneal, narrowing posteriorly to obtuse tip; about half the length 

 of pygidium; divided into three segments, anterior one consisting of pair of small, 

 oval, triangular lobes, not touching in middle ; the second segment is marked off 

 from the third by transverse furrow, and lias a median tubercle at the posterior end 

 of a faint central ridge ; third or terminal segment shorter than second, and not 

 invaded by median tubercle. Axis circumscribed by strong furrow. Limb as in 

 head-shield, but narrowing anteriorly on each side owing to shape of axis. Border 

 as in head-shield, but provided on posterior lateral margins with a pair of short, 

 backwardly projecting spines. 



Remarks. — The specimens from Balclatchie, which Nicholson and Etheridge 

 figured (M., 1 fasc. h, 1879, p. 200, pi. xiv, fig. (3) as A. agnostiformis, are quite 

 distinct from typical examples of that species, and belong to A. girvanensis. 

 The figure given by these authors is a composite restoration drawn from three 

 specimens — a fact noted by them on the tablet bearing the fossils, though not in 

 the description of the plate. The figure, however, fails to give a correct idea 

 of the characters of this form, which is more precisely diagnosed in the preceding 

 description. 



Affinities. — The transverse groove across the glabella recalls the similar feature 

 in .1. maccoyi, Salter; 2 but in the latter species there is no median tubercle, and 

 the glabella generally expands anteriorly. The faint radiating lines across the 

 limb also remind us of this species, but the small subtriangular areas at the base of 

 the limb are apparently absent. The shape of the glabella and the absence of a 

 transverse groove and median tubercle distinguish A. agnostiformis from our 

 Grirvan form. 



The description of the thorax which is here given is based on one individual 

 having one thoracic ring nearly completely preserved. The short broad pleura-, 

 grooved and notched at the ends, recall ,1. princeps, Salter, but the axis with its 

 lateral nodules seems more like J. tardus, Barr. 



The pygidium has an axis relatively rather longer and narrower than A. agnosti- 

 formis ; the median tubercle forms the swollen posterior extremity of a longitudinal 

 low ridge, and herein also differs from the latter species; but in the segmentation 

 of the axis it agrees. The posterior segment in the typical examples of 

 A. maccoyi is much longer. There is also a pair of short lateral spines on the 

 border, which is broad and rounded and separated from the limb by a broad sulcus, 

 as in .4. agnostiformis. There is thus seen to be a combination of the characters 

 of A. maccoyi and A. agnostiformis, with some others not found in either. But, on 

 the whole, the affinity seems closer with the former than with the latter. 



In A. glabratus, var. ingrica, Schmidt, 3 there is on the axis of the pygidium a 



1 The letter " M " is used to signify the ' Monograph of the Silurian Fossils of the Grirvan District.' 



2 Salter, ' Mem. Geol. Surv./ dec. xi (18(34), pi. i, Eg. (<. 



:i Schmidt, 'Kev. Ostbalt. Silur. Trilob.,' pt. iv, p. DO, pi. vi, tigs. 39—43. 



