236 TRANSACTIONS OF THE [may 18, 
sometimes only two, can be discerned. All the furrows are 
usually indistinct or even obliterated in the later species of the 
Paradoxides Beds, in the adult shields; but even in these there 
are often impressions of these furrows on the inner surface of 
the test. 
It is in the species of the Olenellus Zone that the series of fur- 
rows marking the somites is most clearly seen. JW. speciosus for 
instance has three pairs of broadly, but distinctly impressed fur- 
rows on the sides of the glabella and also a pair of sharply cut 
occipital furrows.* In M. lobatus the furrows of the glabella 
are not quite so obvious, and the posterior of the three is usually 
very indistinct, but the number is the same as in the preceding 
species. 
Both Agnostus and Microdiscus exhibit larval characters in 
the anterior lobe of the glabella, the former by the large size 
of the anterior lobe (as in Regii) or its distinctness (as in Fal- 
laces and Longifrontes). In Microdiscus, however, the larval 
facies is maintained by the large size of the anterior lobe as_ 
compared with those behind it. The Ptychparidz in the Pro- 
taspis stage, which the author has studied, agree in having the 
somites of the cephalic axis short except the front one which is 
much larger and longer than the others; as regards the length, 
this is a permanent characteristic in the typical Microdisci of 
the Olenellus zone, for in these the front lobe is nearly as long 
as the other three.t ; 
As in Agnostus, and in many other trilobites the pygidium 
of Microdiscus is divided into an anterior and a posterior part, 
the latter distinguished by the imperfect segmentation of the 
rachis ; this is very well seen in the section (Eodiscus) described 
hereafter, wherein the protopygidium is marked by a tubercle on 
the summit of each ring of the rachis, while on the neo-pygi- 
dium there is a row of low tubercles along the crest of the ring, 
similar to those on the lateral lobes (of M. pulchellus, etc.). 
The pygidium in this group by the appearance and subsequent 
effacement during growth of costz on the side lobes shows the 
addition of somites beyond the number of those of the early 
stages ; while in other sections as Dawsonia (see below) the sur- 
face moulding shows the cementing of additional segments to the 
front of the pygidium.{ 
A general rule holds in the genus Microdiscus as to the in- 
crease of the number of rings in the rachis of the pygidium, ac- 
cording to the geological age of the species; the majority of those 
*On many heads of adult examples of JZ. speciosus all these furrows are quite ob- 
literated. 
+M. speciosus and M., lobatus, but this is not the case with MW, Schucherti in which 
this lobe is short as in the Microdisci of the Paradoxides beds, 
tSee also the pygidium ascribed to M. precursor. 
