RAYMOND AND BARTON: AMERICAN SPECIES OF CERAURUS. 531 



outer portion consisting of the free ends of the pleura. Along the 

 line dividing these parts, is a series of prominent tubercles, one on 

 each pleuron. In front of each tubercle in this line there is a process 

 which extends under a small fold in the shell of the pleuron in front — 

 the well known fulcral arrangement in trilobites having the power of 

 enrollment. On the inner part of the pleural lobe, the pleura are 

 crossed by deep diagonal furrows which divide this part of each pleuron 

 into convex, triangular pieces. The diagonal line starts at the dorsal 

 furrow on the anterior margin of each pleuron, and runs backward and 

 outward. The nodes thus formed are hollow beneath, and their 

 purpose is evidently to strengthen the shell. 



The pygidium is small and short, made up of four ancylosed seg- 

 ments, the first pair of which end in very long, curved, divergent spines. 

 The part of the pygidium within the spines is rounded in outline, and 

 marked by two pairs of ribs which cross the axial portion without a 

 break. The form of the ribs is controlled by the parabolic curve of 

 the great spines, and the third pair make almost a semicircle. The 

 fourth segment is represented by a single ridge running backward on 

 the axial line, within the semicircle formed by the third ribs. Between 

 each pair of ribs are a pair of deep depressions. The ribs extend to, 

 and sometimes cross the narrow convex border. When they cross, 

 they make two pairs of short spines within the great spines, while the 

 central unpaired rib makes a median spine. These spines are never 

 long, and sometimes, when present, they are so short that they merely 

 roughen the outline of the border. 



Variations. — The amount of variation among specimens from 

 Trenton Falls is surprisingly slight. As has already been noted, the 

 position of the eye is very constant, and other features, such as the pro- 

 portion of length of cephalon to width, rate of tapering of glabella, 

 and form of nodes on the thorax, are about the same in all specimens. 

 Moreover, young specimens show very little difference from adults. 

 The smallest specimen at hand, 3 mm. long, has the same number of 

 segments as the adult, the same spines on cheeks and pygidium, the 

 same glabellar furrows, and the same nodes on the thorax. In small 

 specimens, the spines on the pygidium, seem, as a rule, to be more 

 curved, and to have their tips nearer together than in the adult. 

 The average length for specimens at Trenton Falls seems to be about 

 30 to 35 mm., and the majority of the specimens in the collection are 

 shorter than this. The largest specimen is 58 mm. long, and there 

 are few fragments indicating specimens of larger size. It is important 

 to note that the large specimens have the eyes as far forward as the 

 smaller ones. 



