238 TRANSACTIONS OF THE [May 18, 
The embryonic tests of individuals of the section Eodiscus 
do not show any characters special to the genus, but these 
speedily appear in very small larval tests, and are first recog- 
nizable in the headshield. In general outline and in having 
costal ribs on the sides of the pygidium, this part of a young 
Eodiscus is much like that of a Ptychoparia at the corre- 
sponding stage, but in the later moults the distinctive features 
of Microdiscus appear; the rings of the axis become more 
numerous, the pleural lobes inflated and the form of the 
pygidium more elongated. In the species of this section 
(Hodiscus) the furrow on the pleure (of the posterior joint at 
least) run forward and outward as in Agnostus; this is per- 
haps due to the strong geniculation of the anterior marginal 
fold of the pygidium, necessitating a convexity of the overlap- 
ping pleura. 
The genus Microdiscus serves as a link between the Paradox- 
ides and the Olenellus faunas, and there is a gradation of re- 
sembling species through two lines of development, one with 
few, the other with numerous joints in the rachis of the pygid- 
ium. Though Microdiscus is so common in the Olenellus Fauna 
and passes into that of Paradoxides, no example has been found 
in the Protolenus Fauna. 
Micropiscus ScHUCHERTI n. sp. Plate xvii., figs. 4 a and b. © 
Headshield subcircular, one-quarter wider than long, flattened 
in front, drawn in and upturned at the genal angles; axis pro- 
jecting behind. Anterior marginal fold enclosing a flattened 
area, which is indented in front of the glabella. Glabella two- 
thirds of the length of the shield, cylindro-conical, depressed in 
front, elevated and projecting backward behind; three pairs of 
furrows faintly impressed on the sides. Occipital ring not vis- 
ible. Cheeks moderately arched, not meeting in front of the 
glabella. Posterior marginal fold wide at the outer end, shield 
narrower here than opposite the cheeks. 
Thorax not known, but the relief of the approximate margins 
of the two shields indicates a prominent rachis and strongly 
geniculate pleurz, with furrows directed forward. 
Pygidium longerin proportion than the head shield, and more 
decidedly arched. Rachis nearly three-quarters of the length of 
the shield; it has about six joints, the anterior four prominent 
and each crowned with a tubercle. Side lobes sloping down to 
a flattened marginal fold, which is strongly geniculated in front, 
with an almost vertical outer slope. 
Sculpture.—Surface of both shields minutely granulate. 
