THE FAUNA OF THE ST. JOHN GEOUr. 109 



The posterior margin is now straighter and the furrow heavier. 



The cheeks are now more like those of the adult in form, and the little spines at the 

 corner more distinct. 



Sculpture. — The spines on the cheeks are now visible with a lens, and the rows across 

 the glabella also. 



Fourth Stage [Fig. 18.) — Shield, 3J X 5J mm. Oblong semi-elliptical, without 

 spines. 



The anterior marginal fold is now traceable all around, and is wider and stronger every- 

 where than in the younger stages. 



Glabella much wider behind than before ; nearly one-third of the transverse diam- 

 eter. Three pair of furrows are visible, two directed backward, as in the adult. The 

 frontal lobe is rounder than in preceding stages, having gained in length. 



The cheeks have become ovoid and spread ax)art at the base, owing to the flattening of 

 the posterior outer angle and the widening of the base of the glabella. 



Sculpture. — Tubular spines, are now visible on all projecting i?arts,— a double row on 

 the front third of the anterior marginal fold, and a single row on the rest of the fold ; there 

 is also a row of about five s^ilnes on each limb of the posterior marginal fold. 



Fifth Stage (Fig. 19.) — Shield, 5X8 mm. Narrowly semi-circular, somewhat angu- 

 lated, without spines. 



The Glabella and its furrows are more distinct than in the preceding stage. 



Posterior margin. — Notch in the outline at the inner end very distinct ; fold thickened 

 toward the outer end, and rounded forward at the genal angle. 



Sculpture. — All elevations are now covered with spines, and the number does not differ 

 greatly from those on some adult individuals, but they are arranged with more regularity, 

 especially the rows across the glabella and frontal lobe ; those around the outer base of the 

 cheek may also be seen to be roughly arranged in rows parallel to the anterior border. 

 The average number of spines on heads of indiA^duals of this age is given in the preceding 

 table. 



From this stage onward to the adult period there are no very decided characters mark- 

 ing the i)rogress of the animal toward maturity, but the changes in the proi^ortions of dif- 

 ferent parts of the shield are gradual. Of certain sizes, there is a predominating number 

 of shields preserved, as, for instance, at 6 X 10 mm., Ï X 12 mm., 9 X 1*7 mm., [Fig. 20) ; 

 10 X 20 mm., 14 X 28 mm., (Fig. 21) ; 19 X 38 mm., (Fig. 6.) The form of the .shield 

 changes during this time, so that the width becomes double the length. 



Size 9 X 1*7 mm. (Fig. 20 and 9.) — Of this size, an individual with other parts attached 

 to the shield has been obtained, from which it appears that the genal sj)ines have become 

 shorter in proportion to the size of the shield than they were at first. Four segments of 

 _ the thorax attached show a rapid narrowing of the body-rings at this stage. 



Size 10 X 20 mm. — At this period the head-shield, as preserved in the slates, is about 

 twice as wide as long, and continues to maintain this width until the adult size is reached ; 

 the genal spine is also short, as in the adult. 



Size 14 X 28 mm. (Fig. 21.) — From this period to maturity there is greater variation 



