1 14 [Senate 



we have indicated the characters common to the genera which constitate it. Not- 

 withstanding their affinities, it is easy to recognize at a glance that the Pabadox- 

 IDES are distinguished by the large number of segments, the form of ribs, the hy- 

 postoma, the great prolongation of the cephalic limb, the very elongated eyes, and 

 the general appearance. The only type where it is difficult to establish a strong 

 line of dcmarkation is Olenus; particularly when we consider P spinulosus, 

 which approaches nearest to Paradoxides, and which Burmeister has classed . 

 with them. Not having at our disposal the materials necessary to show fully the 

 distinction between these two genera, "we will conline our remarks to, 1, That in 

 Olenus, the glabella has a form constantly narrowing towards the front, and 

 whichcontrasts with those of the Paradoxides; 2, the lateral furrows of Olenus 

 are very much inclined, and rarely unite in pairs on the axis, while in Paradox- 

 ides they are almost horizontal, and the two last pairs generally form two paral- 

 lel branches traversing the glabella; 3, the number of thoracic segments in the 

 first genus appears not to exceed fifteen or sixteen, which is the least number ob- 

 served in the second; 4, the pygidium of Olenus usually differs from that of 

 Paradoxides by a greater relative development of the lateral lobes. We hope that 

 the Swedish savants will be able to define the limits between the two genera. The 

 discovery of the hypostoma of Olenus would contribute much to attain this 

 end."* 



In comparing our own species with Olenus, we find some differences in the form 

 of tlie cephalic shield, but more particularly in the form of the glabella; which, 

 however, from imperfection in the specimens, does not admit of minute compari- 

 sons. Our specimens have no more than thirteen or fourteen segments of the thorax 

 (and the one referred to Peltura has eleven), instead of fifteen or sixteen, and 

 the direction of the lateral furrow is different. The greater development and ex- 

 tension of the third segment of the thorax is a remarkably distinctive character, 

 and the same feature is shown in the posterior segments of one species. The form 

 and development of the p3^gidium also differs from that of Olenus, in the lesser 

 lateral expansion, and absence of segments on the lateral lobes. 



When we compare with Paradoxides, we find the cephalic shield proportionally 

 broader and shorter, while there is no expansion of the glabella towards the front; 

 nor do the transverse furrows extend entirely across this part, except at its base. 

 This feature and the facial suture, though indistinct, correspond more nearly with 

 Olenus. 



The smaller number of thoracic segments is a distinguishing feature, and the 

 direction of the segment-furrow differs essentially. In one feature, that of the 

 greater development of the third segment, it corresponds with Paradoxides, 

 where the second segment has a greater development than the others. In the ex- 

 treme development of the posterior segments, in one species, there is likewise a 

 similarity Avith Paradoxides. In the slight development of the pygidium, our fos- 

 sil corresponds in some degree with Paradoxides. 



In the perfect condition, one species appears to have been furnished with a row 

 of nodes or spines along the dorsal ridge. 



We have, therefore, although the material is imperfect, the means of showing 

 well-marked distinctions between these forms and the allied Genera Olenus and 

 Paradoxides. * 



The species referred to Peltura has the cheeks separated, and therefore there 

 is some obscurity about the suture margins. The expansion of the lateral lobes of 

 the thorax is imperfect in the specimen figured; but, from examination of this and 

 other specimens, the third segment does not appear to have been prolonged as in 



* Barrande, Systeme Silurien du centre de la Boh- me, Vol. i, p. 367. 



