pM BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
spines, those of the first segment being the longer. Barrande figures 
an entire specimen of P. inflatus Corda, 5 mm. long, which differs 
from H. carens only in possessing free cheeks, glabellar furrow No. 3, 
and in having the terminal spines of the first thoracic segment reduced 
to normal, while the second pair have increased in length. Into what 
species this form finally developed there is no way of determining with- 
out more material. It is interesting to note, however, that the devel- 
opment agrees with the series described above in that the glabella 
is first smooth, and the glabellar furrows are added during the nepionic 
stages. ‘They seem to be greatly retarded in this form, as only two 
furrows (Nos. 3 and 4) have been formed in the largest specimen fig- 
ured by Barrande. It may be noted that in this form, as in Hydro- 
cephalus saturnoides, the brim on the cranidium widens constantly 
during the known stage of growth, though it never achieves any great 
width. The M.C.Z., contains a single specimen of P. inflatus 
(No. 651) about 5 mm. long from Velka in the Cambrian band of 
Ginetz. All the other specimens recorded have come from the band 
of Skrey, but that it does occur in the more southern band suggests 
that it may possibly be the young of some form which in the adult 
has only two pairs of glabellar furrows, possibly P. bohemicus (Boeck). 
The M. C. Z. contains a single minute specimen (No. 33) 1 mm. long 
of a young “Hydrocephalus” which is in many respects quite unlike 
H. saturnoides. (Plate, fig. 8). The specimen differs from that 
form in having the glabella narrow, expanding forward. The occipital 
ring and furrow are well marked, the glabellar furrows 2, 3, and 4 are 
deeply impressed, extend across the glabella and divide it into ring- 
like lobes. The anterior lobe, which is composed of lobes 1 and 2, is 
transversely oval, and nearly twice as wide as that portion of the 
glabella back of it. Furrows 1 are faintly indicated, and the frontal 
lobe has a deep longitudinal furrow which does not reach lobe 3. The 
palpebral lobes are long, and extend into spines behind the occipital 
segment. The brim is narrow, and the truncation at the sides is so 
slight that it is doubtful if free cheeks were present. This specimen is 
about the size of the smallest specimen of H. saturnoides, and it does 
not seem that it could represent a younger stage in the development 
of that species, but it appears probable that it is the protaspis of 
another form. From the outline of the posterior end of the specimen, 
it seems probable that it is complete, and that the small posterior pro- 
jection represents the proto-pygidium. | 
The young specimen of P. spinosus figured by Barrande has already 
been alluded to. Its chief interest lies in the fact that so small a 
