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 



3 segments 



4 segments 



5 segments 



6 segments 



7 segments 



8 segments 



specimen, 3 mm. long 



specnnen, 6 mm. long: 



2 specimens, 3.5 and 5 mm. long, respectively: 



1 specimen, 5.5 mm. long: 



1 specimen, 7 mm. long: 



1 specimen, 8 mm. long: 



1 specimen, 6.5 mm. long: 



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 a specimen 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 mitogcny. — Basilicus 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. 



Ogygites stage. The glabella soon becomes flattened, and the 

 facial sutures intramarginal, as in the adult of Ogygites. 



