Crijptozoon Minnesotense at Northfield — Chaney. 283 



all the characteristics of the smaller masses which are regarded as 

 undoubted fossils. The difficulties in regarding the entire mass as 

 fossil are: 1st. The size of the mass. If they are single fossils 

 they are entirely unique as to size. It is true that corals have pro- 

 duced masses fully as large, but a colonial protozoon reaching any- 

 thing like such dimensions is unknown so far as I can discover. 

 2nd. The change in structure which is observable as we pass from 

 the center toward the outside. There appears to be no good reason 

 why any such change should occur. If they are entirely organic 

 there should, it would seem, be a substantial uniformity in structure 

 from center to periphery. 



The supposition that we are dealing with a simply concretion- 

 ary structure seems untenable in view of the opinion of those who 

 have made the most careful examinations. 



The microscopic structure is regarded by Prof. N. H. Winchell 

 as conclusive evidence of organic origin. Prof. Henry M. Seely 

 writes me that he has discovered in the Calciferous of the Cham- 

 plain valley two forms similar to those described and which he 

 names provisionally Cryptozoon steeli and C. saxirodeum. C. steeli 

 varies in size from two inches to two feet, while C. saxirodeum 

 rarely exceeds nine inches. It will be noticed that the Northfield 

 specimens are immensely larger than either of these. Professor 

 Winchell in his report has given to the forms found at Northfield 

 and Cannon Falls and varying from two inches up to twenty-five 

 inches in diameter, the name Cryptozoon minnesotense. Should it 

 seem that these larger forms are specifically distinct, C. giganteuin 

 would certainly not be an inappropriate name. 



The third possible view -suggested, namely that these structures 

 are concretionary about an organic core, has several points in its 

 favor. 1st. It accords with many other well known cases- 

 Nothing is of more frequent occurrence than to find a concretion 

 having a well preserved fossil in the centre and the dimensions of 

 the concretion having no particular relation to the size of the fos- 

 sil around which it has formed. 2nd. The marked difference be- 

 tween the inner and outer layer may thus be accounted for. The 

 change in structure appears to be rather gradual, but in a few cases 

 there is a somewhat clearly defined boundary between the central 

 core and the surrounding layers. 3rd. The structure of the outer 

 layers so far as can be judged from observation with a lens is sug- 

 gestive of concretionary rather than of organic origin. 



