252 bulletin: museum of comparative zoology. 



and .80. In specimens with cephala over 30 mm. long, the growth 

 seems to be more rapid in the transverse direction, and the ratio drops 

 again. It is only .62 in the two largest specimens in the collection, 

 the largest cephalon being 60 mm. long. The form in these large 

 specimens remains subtriangular, and does not return to the semi- 

 circular outline of the young. 



Pygidia show a similar series of changes, but the index rises higher, 

 and does not show so great a falling off in large specimens. In the 

 adults, the pygidia are usually of about the same length, or a little 

 longer, than the cephala, but are always narrower, hence the higher 

 index. In specimens where the length of the cephalon is .75 the width, 

 the length of the pygidium is generally .80 the width. In the larger 

 specimens, where the indices of both shields have begun to drop, the 

 difference is more noticeable, the index of the cephalon being .62 while 

 the index of the pygidium is .77 in one well-preserved specimen. 

 Raymond and Narraway found occasional specimens in which the 

 length equaled the width, but in the present collection the highest 

 index noted was .85. 



In small specimens, the pygidium is about one half as long as the 

 cephalon, and the two shields do not reach quite equal length until 

 the whole animal has attained a length of about 50 mm. In specimens 

 more than 100 mm. long there is a tendency for the pygidium to exceed 

 the cephalon in length, the difference being as much as 10 mm. in 

 some large specimens. That the pygidium does, in the adult, finally 

 reach the same length as the cephalon, seems to be due, not to an 

 acceleration in growth at any particular time, but rather to a retarda- 

 tion of the longitudinal growth of the cephalon, during the process of 

 widening. The pygidium never becomes quite as wide as the cepha- 

 lon, so that more energy can be put into longitudinal growth. 



Glabella and axial lobe of the pygidium. — The glabella soon loses its 

 convexity and prominence, as well as its furrows, basal lobes, and 

 median tubercle. These features are still visible on a cephalon 4.5 mm. 

 long, but are faint, and the front of the glabella no longer rises abruptly 

 from the brim. The outline of the glabella is still distinguishable on a 

 cephalon 7.5 mm. long, but it is there hardly more convex than in the 

 adult. Something of the form of the glabella can be made out on 

 most specimens, whatever the size, but it is rather smoothly merged 

 into the general surface of the cephalon in all specimens with a cepha- 

 lon more than 8 mm. long. 



The axial lobe of the pygidium remains convex and conspicuous 

 somewhat longer than the glabella does. On a specimen 9.5 mm. 



