RAYMOND: ISOTELUS GIGAS DEKAY. 251 



pygidium, 2 mm. long, shows the furrowed ribs, but less strongly, 

 and pygidia 3 mm. long are quite smooth, except for the rather promi- 

 nent axial lobe. The pygidia of young specimens are much smaller 

 than the cephala, being both shorter and narrower. 



Summary of characters of specimens less than 5 mm. long. The 

 cephalon makes up one half the entire length, and has a greater area 

 than all the remainder of the shell ; it is semicircular in outline, with a 

 wide concave brim, and long genal spines. The eyes are very large, 

 close to the glabella and to the posterior border. The glabella is 

 convex, prominent at the front, narrow behind, and expanding for- 

 ward, with a median tubercle between the isolated lobes at the posterior 

 end. The free cheeks apparently do not meet in front. 



The thorax consists of two or three segments; the axial lobe is less 

 than one fourth the total width. The pleura are flat to the fulcrum, 

 which is far out, and are there turned down abruptly. Each pleuron 

 bears a straight furrow, which extends beyond the fulcrum. 



The pygidium is small, semicircular, with narrow, prominent axial 

 lobe, and narrow, concave border. The pleural lobes are crossed by 

 distinct, furrowed ribs, and there are rings on the anterior end of the 

 axial lobe. 



Changes duriyig development. — It will be shown in the sequel that 

 some of these youthful characters are lost much more quickly than 

 others. The first to be changed is the marginal position of the facial 

 suture, and the most persistent is the genal spine. The ribs on the 

 pygidium disappear very early, while new segments are still being 

 added to the thorax. Then the glabella becomes flattened, the axial 

 lobe becomes wider, the eyes relatively smaller, the genal spines shorter, 

 and the cephalon more triangular and smoother. 



Outline of cephalon and pygidium. — The outlines of the shields, and 

 their changes, were quite fully discussed by Raymond and Xarraway, 

 but the present more complete material has furnished some additional 

 facts. In the smallest specimens so far seen, both the cephalon and 

 pygidium are one half as long as wide, and regularly semicircular. 

 In the cephalon, this condition persists until this shield reaches a 

 length of about 4 mm., when the ratio of length to breadth begins to 

 rise, and at the same time the sides of the cephalon become straighter, 

 so that the general form becomes subtriangular, instead of semicircu- 

 lar. This change takes place rapidly, so that a cephalon 12 mm. long 

 is three fourths as long as wide, and few specimens of any size have a 

 higher ratio than this. Most cephala whose length is between 12 mm. 

 and 30 mm. have this ratio (length divided by width), between .72 



