PELTURA OLENOIDES. 10] 



close to the anterior end of the glabella. Facial suture running backwards 

 and slightly outwards from the anterior margin to the eye and thence in a gently 

 sigmoidal curve to the posterior lateral angle. Fixed cheeks triangular, about 

 half the width of the glabella at the base, sloping gently downwards from the 

 axial furrows. Free cheeks crescentic, narrow, with a narrow raised marginal 

 rim separated by a marginal furrow. 



Thorax consisting of 11 (?) segments, widest about the sixth segment. Axis 

 nearly twice as wide as the pleurae in front, tapering rapidly in the hinder part 

 of the thorax, where its width is but little greater than that of the pleura. 

 Pleuras very short in front, attaining their greatest length about the middle of 

 the body ; fulcrum placed very near the axis, deeply grooved, obliquely truncate 

 but terminating in short points. 



Tail obscure, but appears to form a small arc of a circle, with a wide axis, 

 and a distinct marginal limb defined by a marginal furrow. 



Dimensions. — Mr. Williams' specimen, allowing for the missing part of the 

 tail, must have been about 16 mm. long. The type-specimen belongs to an 

 individual about twice as long. 



This species was founded by Salter on an imperfectly preserved head. It 

 clearly belongs to Peltura, but is at once distinguished from the other British 

 forms of the genus by the absence of any trace of glabellar furrows. The shallow 

 neck-furrow arching forwards in the middle also appears to be characteristic. 

 It should be remarked that this furrow may not be homologous with the true 

 neck-furrow, but more probably represents the curving groove which in many 

 trilobites, including Peltura itself, divides the neck-segment into a central posterior 

 portion and an anterior lateral lobe on each side. 



Mr. Williams' more nearly complete specimen (Plate XII, fig. 5) comes from the 

 same horizon and locality as Salter's, and there can be little doubt that it belongs 

 to this species. It shows the same shallow neck-furrow arching forwards in the 

 middle line. Unfortunately it is much distorted, and it is not certain where the 

 thorax ends and the tail begins. But the presence at the hinder end of the 

 specimen of what seems to be clearly the marginal limb of the tail, indicates that 

 there are not more than eleven segments in the thorax. It is even possible 

 that the eleventh segment may belong to the tail. 



Probably the nearest ally is Brogger's " Cyclognathus " costatus, 1 in which the 

 glabellar furrows are obsolete and the tail is entire with a well-defined marginal 

 limb. So far as it is possible to judge, the proportions are approximately the 

 same, and it is impossible to point to any definite difference between the two 

 forms. But the very imperfect condition of our specimens prevents detailed 

 comparison. 



1 Die Silur. Etagen 2 und 3, p. 110, pi. i, figs. 5 a— d ; pi. ii, figs. 10, 11. 



