RAYMOND: ISOTELUS GIC, AS DKKAY. 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 boi-der. 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 na,rrower 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. HI), 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 al)ove, one may cite the condition seen in certain species of 

 Onchometopus, especially 0. 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. Further, 

 all the strongly segmented asaphids, like Basilicus, Ogygopsis, Ogygio- 

 caris, and Ogygites, have more or less of a border, and it is only the 

 rather smooth forms, like Nileus, Asaphus, Onchometopus, and the 

 like, which lack it. 



Genal spines. — The reduction of the genal spines during life was 

 discussed 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 p\gidium. The 



