254 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
smallest specimens do not seem to have the spines quite so-long, but 
this may be an accident of preservation. A specimen 8 mm. long has 
spines extending to the front of the pygidium, and one 18 mm. long 
has them reaching the sixth segment. Specimens about 30 mm. long 
show spines extending to the fourth segment, and those 35 to 50 mm. 
long show small, narrow spines reaching the third segment. Speci- 
mens above 55 mm. long do not show any spines at the angles, except 
in the case of a few specimens, which retain them as youthful charac- 
ters are sometimes retained by adults of any species. 
Thorax.— The full number of segments is attained when the animal 
is about 8 or 9 mm. long, and the following are measurements of speci- 
mens with less than 8 segments: — 
2 segments; specimen, 3 mm. long: 
3 segments; 2 specimens, 3.5 and 5 mm. long, respectively: 
4 segments; 1 specimen, 5.5 mm. long: 
5 segments; 1 specimen, 7 mm. long: 
6 segments; 1 specimen, 8 mm. long: 
7 segments; 1 specimen, 6.5 mm. long: 
8 segments; smallest specimen, 9.5 mm. long. 
In the smallest specimens the axial lobe of the thorax is very narrow, 
being only one fifth the total width in the specimen which is 3 mm. 
long. A specimen 5 mm. long has a thorax of three segments, and the 
axial lobe is one fourth the width of the thorax. In aspecimen 8 mm. 
long this ratio has risen to .30 and one 10 mm. long has the axial lobe 
40 of the width of the thorax, while a specimen 18 mm. long has the 
same index. Specimens 29 and 32 mm. long have the index .46, 
and one 38 mm. long has the axial lobe one half the total width of 
the thorax, which is the normal ratio for adults of this species. The 
exact ratio .50 was found in eleven specimens varying in length from 
38 to 130 mm. and the greatest departure noticed in all the specimens _ 
measured was .05, the ratio varying from .45 to .53, the latter index ~ 
being found in a specimen 181 mm. long. 
Summary of the ontogeny.— Bastticus stage. The glabella is 
convex, narrowed behind, with a median tubercle and basal lobes. | 
The brim is wide and horizontal, the cephalon semicircular in outline, | — 
the genal angles produced into long spines. The axial lobe of the — 
thorax is narrow, the pygidium has a prominent axial lobe, and the ~ 
pleural lobes of the pygidium are ribbed. 
OayaiTEs stage. The glabella soon becomes flattened, and the — 
facial sutures intramarginal, as in the adult of Ogygites. 
