148 SILURIAN TRILOBITES. 



and short tail, with axis of few joints ; and sides without ribs. Facial suture marginal in 

 front. Hypostome entire. Labrum — ? North Europe. 



Type, S. palpebrosus, Dalm. 



7. Brachyaspis, Salter, 1866. Short, broad, expanded, wdth moderately broad axis; 

 the head with obsolete glabella ; eyes depressed but convex, remote ; plfeurse rounded ; 

 tail short, with no ribs. Facial suture marginal in front. Hypostome entire. Labrum — ? 

 North Europe. 



Types, A. rectifrons, Portlock. 

 A. leevigatus, Angelin. 



[8. Nileus, Dalman, 1826. Very convex, scarcely trilobed, with very wide axis, and 

 hemispherical lobeless glabella ; eyes very large, remote, depressed, reiiiform ; pleurae 

 rounded; tail short broad, with no trace of axis or ribs. Facial suture marginal. 

 Hypostome entire. Labrum scarcely notched. North Europe. 



Type, N. Armadillo, Dalm.] 



I believe other subgenera might easily be formed, but these are perhaps sufficient for 

 convenient reference. For instance, Asaphus nobilis, Barr., and the several Mid-European 

 species connected with it, are very doubtfully forms of Megalaspis ; and might with 

 advantage be kept separate. Asaph, inyens of the same author does not quite fit with 

 Basilicus ; and so of several others. 



I have noticed briefly the geographical range of each of these subgenera under their de- 

 finitions. The species of all the groups except Isotelus are European ; and even this reaches 

 our north-western shores, but is not found on the Continent. Nileus, Sywphysurus, and 

 Cryptonymus are almost exclusively Scandinavian types; Brachyaspis is North-European 

 only. Basilicus and Megalaspis range over North and South Europe, and Btychopyge 

 appears to have a wider range, extending to North America, and even to India. 



I do not know to what group the smooth-tailed Asaphi of the Andes, described by 

 D'Orbigny, belong. The genus is evidently world-wide. 



Returning over the ground just trodden, it may well be questioned if Nileus should 

 be included in the list of subgenera. It not only has the broad and lobeless glabella, the 

 wide axis to the thorax, and the smooth unfurrowed tail of Illccnus, but it has a nearly 

 entire labrum. It evidently leads the way to that genus. 



But then it is so near Symphysurus, which cannot be separated, except by its inflated 

 glabella, from Cryptonymus, that it is difficult to see how to draw the line. It is not so 

 far removed from Cryptonymus as this is from Isotelus, which last may perhaps be 

 regarded as the central type of Asaphus. 



Brachyaspis follows next ; and from this point the subgenera diverge towards Ogygia. 

 Megalaspis consists of expanded forms, which yet have, on the whole, more resemblance 



