228 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
cephalon occupying three fourths of the length and the thorax and 
pygidium one fourth. The thorax contains three segments. The 
cephalon has three glabellar furrows which cross the entire glabella. 
Three more stages of development are illustrated by Barrande, in the 
last of which the test is about 2 mm. long, the cephalon being a little 
over 1. mm. In the third, fourth, and fifth stages the first two seg- 
ments of the thorax bear backward-directed spines and the intergenal 
spines are still present. The most conspicuous feature of the cephalon 
in stage five is the presence of a narrow, smooth, flat brim on the front 
of the cranidium. ‘This is first seen in stage four and becomes wider 
in stage five. The three glabellar furrows and the median longitudinal 
furrow are still present at stage five, but the median furrow is not SO 
conspicuous as in the smaller specimens. 
The ontogeny of Olenellus indicates that the palpebral lobes are 
formed by the recurved pleura of the second glabellar lobes. It is 
very important to note that in these specimens known as Hydro- 
cephalus saturnoides the anterior ends of the palpebral lobes join the 
glabella in front of the anterior glabellar furrows, thus indicating that 
the furrows present are 2, 3, and 4. Beyond stage five Barrande did 
not trace any line, but the writer believes that Paradoaxides orphanus 
and P. pusillus represent the next stages of this same species. In the 
M. C. Z. there is a cranidium 1.5 mm. long, identified as P. pusillus, 
but answering better to the description of P. orphanus, which in some 
measure fills the gap between the largest of Barrande’s figured speci- 
mens of H. saturnoides and the smallest of his P. orphanus and P. 
pusillus. In this specimen the anterior brim is narrow, occupying — 
about the same proportion of the whole length as is shown in Bar- — 
rande’s figure of P. orphanus. Glabellar furrows 2, 3, and 4 all cross 
the whole glabella, as in H. saturnoides instead of 3 and 4 only, as in 
P. pusillus, but the connection between the two sides on furrow 2 is — 
quite shallow. This specimen, moreover, adds another pair of fur- 
rows at the sides just in front of the palpebral lobes, as in P. pusillus. 
From this specimen to a typical pusillus with a wide brim, the collee- 
tion contains all stages, so there is no doubt of the connection in that 
direction. Barrande has figured (Loc. cit., 1872, 1, suppl. pl. 9, 
fig. 22, 23), an entire specimen of P. pusillus 2.5 mm. long, the cepha-— 
lon of which makes up 55% of the length. Seven free segments are 
present, and the pygidium contains three or four more. The cephalon 
has a wide brim, 23% of the whole length, and there are no intergenal — 
spines present, though the first two segments of the thorax have long 
spines, the spines of the second being longer than the first. _ 
