260 bulletin: museum of comparative zooLOoy. 



than 20 mm. long are in the matter of size and the shortening and 

 thickening of the genal spines. 



Nearly all the specimens in the Dyer collection are either incomplete 

 or enrolled, so that the specimens have to be considered, not in terms 

 of their total length, but in accordance with the length of one of their 

 shields. The smallest specimen in the collection has a pygidium 

 6 mm. long, and the largest pygidium is 128 mm. long. This species 

 differs from I. gigas in that the two shields become equal at an early 

 age, and stay equal throughout the greater part of the adult stage. 

 One adult specimen, (cephalon 46 mm. long), has the pygidium shorter 

 and more nearly semicircular than the cephalon, which is the reverse 

 of what is found in /. gigas. There are not enough good specimens to 

 show whether this is the general rule in this species. According to 

 the measurements of thirteen specimens from Cincinnati, the length of 

 the cephalon averages a trifle less than .6 the width, and the pygidium 

 is usually a little longer, the average being .64. The axial lobe of the 

 thorax averages about .42 of the total width, and there is surprisingly 

 little variation, the extremes being .40 and .44. The axial lobe is then 

 generally a little narrower than in /. gigas. 



Isoiclus maximus from Toronto, Ontario.^ Thvough the courtesy of 

 Prof. W. A. Parks, I have been able to study a series of very fine speci- 

 mens in the collection at the University of Toronto. The specimens 

 were all from the Lorraine in the vicinity of Toronto, and were very 

 well preserved, though generally a little flattened. So far as the 

 writer could determine from the large collection at the University, 

 and from a short experience in the field, Isotchis maximtis is the only 

 asaphid present in the Lorraine at Toronto. The specimens are quite 

 large, ranging from 70 to 285 mm. in length, and they are extended, 

 not enrolled as is generally the case with the specimens from Cincin- 

 nati. They show remarkably short, wide cephalons, the average 

 ratio of length to width of 11 cephalons, 9 of which were over 50 mm. 

 long, being .46, and the range of variation, .43 to .51. The pygidia 

 are in all cases longer than the cephalons, and the ratio for fifteen 

 specimens averages .59. The axial lobe of the thorax also averages a 

 little narrower than in specimens from Cincinnati, the average of 

 fifteen specimens being .374, and the limits of variation .34 to .40. 



The largest specimen which was well enough preserved to yield 

 accurate measurements was 262 mm. long. The cephalon made up 

 .30 of the length, the thorax .33, and the pygidium .37. The greatest 

 width was .69 of the length. The Dyer collection contains a poorly 

 preserved specimen from INIorrow, Ohio, seven of whose thoracic 



