208 TRANSACTIONS OF THE [may 18, 
others by features held in common. The abortion of the pos- 
terior lobe of the glabella, the absence of the occipital ring, the 
immutable number of two joints in the thorax, all show the 
divergence from the common type; and also there is seldom 
seen on an Agnostus the proof of that development and ex- 
pansion of the pygidium which can be traced in the moulding of 
the surface of this shield in most other primordeal trilobites. 
Nevertheless there are some indications of a closer resem- 
blance between Agnostus and its allies in the earlier stages of 
growth than appear in the adult trilobite. Owing to the min- 
uteness of the shield in the young of this genera, it is difficult to 
make out the outline and contour of the test in the early stages ; 
but it may be said that the pygidium is proportionately broader 
and both the marginal fold and furrow wider than is found to 
be the case at a later period; in the early larval stages it thus 
approximates more in form ard contour to other trilobites than 
at a later period. 
By comparing different species of Agnostus, and by a close 
examination of the surface features of the tests, we arrive at a 
few indications of the process by which this peculiar self-con- 
tained genus has been developed. 
To look at some Agnosti (e. g. Regii*) one would hardly 
suspect that there are more than two lobes in the glabella, bet 
on examining others (Fallaces and Longifrontes) it will be seen 
that there has been an abortion of the true posterior lobe (called 
‘“‘ basal lobes”), so that in the Regii it has become a mere ridge. 
The so-called posterior lobe of the glabella is also itself in some 
species seen to be indented by two pairs of furrows; hence 
there are normally five lobes in the glabella. 
The abortion of the true posterior lobes of the glabella (the 
‘basal lobes’) is the fixation of an early larval condition in 
Agnostus. This lobe in Agnostus never becomes an integral 
part of the glabella, but consists of two small triangular lobes 
(sometimes nearly obsolete) below the level of the glabella and 
connected behind it by a narrow thread-like ridge. The trian- 
cular form of these lobes is not embryonic, it shows a larval 
development up to a certain point; but its sub-ordination to 
the glabella and its separation from it, is evidence of subse- 
quent arrest of growth. 
The suppression of the occipital ring reduces the cephalic 
somites in Agnostus to five, which is the complete number in 
many trilobites which have this ring. In most species of 
Agnostus the occipital ring is not recognizable, hence the first 
*The several sections into which the genus Agnostus has been divided by Tullberg 
are described at page 204. 
