RAYMOND: ISOTELUS GIGAS DEKAY. 253 
long, where the glabella is quite flat, the axial lobe of the pygidium is 
prominent, even at the posterior end, which rises abruptly from the 
flattened border. It would be expected that the pygidium would 
retain its youthful characters longer than the cephalon, for it is really 
younger, and has had less time to change. On a specimen 18 mm. 
long, (pygidium 5.5 mm. long), the axial lobe is fairly prominent, but 
does not extend quite to the border, and is less convex throughout its 
length than in the smaller specimens. It is of course, more or less 
distinguishable on specimens of all sizes, but is very faint on pygidia 
more than 25 mm. long, except under certain conditions of preserva- 
tion. The axial lobe also grows shorter as the shell increases in length. 
In young specimens it reaches and overhangs the flattened border 
while in large individuals the posterior end is at a distance in front of 
the border equal to the width of the border itself. 
Border.— The border on the cephalon and pygidium, which is very 
wide in the young specimens, becomes much narrower with further 
growth. In very young specimens its width is equal to one fourth the 
length of the cephalon, but in the adult it is only about one sixth 
the length. In the young the plane of the brim is horizontal, but in 
older specimens it becomes gradually more inclined and less concave. 
This fact has an important bearing upon the relative primitiveness of 
the asaphids with a border and those without. In a previous paper 
(Trans. Roy. soc. Canada, 1912, ser. 3, 5, sect. 4, p. 111), I have con- 
sidered the absence of a depressed border to be primitive in this family, 
but evidence seems to be accumulating that the contrary is the case. 
Beside the above, one may cite the condition seen in certain species of 
Onchometopus, especially O. simplex, and an undescribed form from 
the Eden at Cincinnati. In these species, the pygidia, which, as has 
just been shown, have a tendency to lag behind the cephala in develop- 
ment, often have a very decided trace of a depressed border, as though 
they may have been developed from an Isotelus-like form. F urther, 
all the strongly segmented asaphids, like Basilicus, Ogygopsis, Ogygio- 
caris, and Ogygites, have more or less of a border, and it is only the 
ather smooth forms, like Nileus, Asaphus, Onchometopus, and the 
like, which lack it. 
Genal spines.— The reduction of the genal spines during life was 
diseussed at length by Clarke (Pal. Minn., 1897, 3, pt. 2, p. 704), and 
the present collection confirms the previous statements. The smaller 
specimens, however, show that the spines of the young were even 
longer than was supposed, for specimens 3.5 to 5 mm. long bear spines 
which extend back as far as the posterior end of the pygidium. The 
