Vol. 58. ] MR. F. R. C. REED ON THE GENUS LICHAS, 63 
occipital lobe or ring falls into its right place as the second maxillary 
lobe. 
But the objection may reasonably be urged that this correlation 
leaves out of account the so-called ‘ basal’ lobes which are found 
in many members of the Lichadide. Their presence, however, 
does not upset the above conclusions, because there is considerable 
evidence that these ‘ basal’ lobes are genetically of the nature of 
occipital lobes, and belong to the occipital or second maxillary 
segment of the cranidium. They have usually been regarded as 
belonging to the glabella, and as true third lateral or basal lobes : 
but, if this be the case, it is difficult to see the cause of the peculiar 
course of the occipital furrow in the majority of those species 
which possess them. The course of this furrow closely resembles 
that of the same furrow in the species of Proetus, Cyphaspis, etc., 
which possess unquestioned occipital lobes that have been cut 
out of the sides of the neck-ring by the formation of new oblique 
furrows. The regular course of the occipital furrow, in those species 
of Lichas which do not possess these lobes, is with difficulty 
explicable on the supposition that they are true ‘third lateral’ 
glabellar lobes. The narrowness of the neck-ring behind these 
lobes and its much greater width in the centre, the bending-forward 
of the lateral portions of the occipital furrow in front of them, and 
the non-continuation of this furrow into that which marks off the 
occipital segment on the cheeks, find their counterpart in species of 
Proetus (as, for example, Pr. bohemicus, Cord.) in which occipital 
lobes occur. 
The simplest and least modified condition of the neck-ring, in 
which it is very plain that these so-called ‘basal’ lobes really 
belong to the occipital segment and have been cut out of it, is found 
in those members of the Lichadids in which the median portion 
of the occipital furrow and the so-called ‘third lateral’ or ‘basal’ 
furrows are in the same straight line and make practically one 
simple transverse furrow (as, for example, Lichas verrucosus, Hoplo- 
lichas tricuspidatus,etc.). In many species(Platylichas margaritifer, 
Inchas St.-Matthie, L. triconicus, L. furcifer, etc.) the idea of 
regarding these lobes as belonging to the glabella would never 
have arisen, if the more highly modified examples of them in other 
species had not been at first so interpreted. In the case of Proetus 
there are some species that possess occipital lobes, and others that 
do not; while in other respects these species may be closely allied. 
We may, therefore, not unreasonably be prepared to find similar 
instances in the Lichadide. 
With the foregoing principles of homology in our mind, we 
may now attempt to discover the principal lines of modification 
along which the evolution of the head-shields of the Lichadide has 
proceeded. 
The archetype of the family may be conceived as having a 
glabella with the normal five annulations, as in other trilobites. 
There must have been, therefore, four pairs of lateral lobes and a 
G@.5,G. 8. No. 229: T 
