Rav.mond-Narraway : Notes on Ordovician TRiLonriES. 55 



mens, and there are almost no traces of rings or ril)s. On pygidia 20 mm. 

 long the axial lobe is very faint indeed. 



The changes on the cephalon can not be so well observed for lack of 

 good material. The outline becomes more triangular and the length and 

 width change in the same way as in the pygidium. The eyes become 

 relatively smaller and move backward, as alread\- indicated, and the dorsal 

 furrows become obscure. The spines at the genal angles become shorter, 

 being 3 mm. long on a specimen with a cephalon about 20 mm. long, and 

 being absent on specimens slightly larger. 



Isotelus gigas is common in the Black River near Ottawa, but most of 

 the specimens are small as compared with those found in the Trenton of 

 New York. The species is less common in the Lowville, but typical 

 pygidia have been found in that formation at Mechanicsville, near Ottawa, 

 at Newport, and on Valcour Island, New York. 



Pygidia less than 3 mm. long have not so far been seen by the writers, 

 but on specimens of that length the segmentation is so faint that it seems 

 improbable that a pygidium of this species 2 mm. long would be so strongly 

 segmented as the pygidia described by Clarke as Gerasaphes ulrichana. 

 (Paleontology Minnesota, Vol. Ill, pt. 2, p. 711, figs. 15, 16). Our studies 

 do not, then, give any support to the suggestion of Miller that Clarke's 

 specimens were the young of Isotelus. Strongly segmented pygidia of 

 small size occur in the Chazy, but they have been found to belong to a 

 species whose pygidia are strongly segmented when fully grown. Very 

 small pygidia of Isotelus, some of them less than 3 mm. long, are common 

 in the Chazy, but they show no more traces of segments than do the young 

 of Isotelus gigas. The development of the species thus shows that the 

 smooth surface was acquired earlier in the phylogeny than the broad axial 

 lobe, and is thus a character of more profound importance. 



Hall states (Paleontology New York, Vol. I, p. 231) that the pygidia 

 of the young of Isotelus gigas are more pointed than in the adult, but our 

 study shows a condition exactly opposite to this, the young pygidia being 

 more rounded. 



Isotelus maximus Locke is in many respects more primitive than Isotelus 

 gigas. An incomplete specimen, a photograph of which is here presented, 

 was about 95 mm. long, and has a genal spine 13 mm. long, which, when 

 complete, reached at least as far back as the middle of the fourth segment. 

 The specimen of Isotelus gigas represented as Figure i on Plate XV is 57 

 mm. long and the genal spine is only 3 mm. long. The pygidium of 

 Isotelus maximus is short and rounded in outline like the young of /. gigas. 



