458 H. 8. Williams—Life History of Spirifer levis. 
characters distinguishing all the individuals from other forms 
of the same genus, together with great variability of the charac- 
ters themselves. In the upper part of the Upper Silurian we 
find the same typical characters, with a greater permanence of 
one or other of the variations; but still, in the variations occur- 
ring later in the Corniferous and Hamilton, we have the main 
type represented with some variations strongly marked and 
seeming to be fixed, but still recognized as varieties simply. 
n the Portage, we see under peculiar conditions a solitary 
race of the type with greatly exaggerated size, a luxuriant 
form, but still presenting the typical characters of the second 
varietal type. 
In the Carboniferous we meet with several well marked 
varieties, but no feature which did not appear in the early 
form except large size, which is evidently a mark of good nour- 
ishment and other good conditions of growth. This latter 
seems to be a character of most of the Carboniferous forms of 
Brachiopods which have lived on from earlier times. There 
may be unknown characters to distinguish these forms, but of 
the characters that are preserved we have evidence that in the 
earliest form, the type, & crispus His. of the Niagara, ete., are 
found all those which afterward appeared in the later repre- 
sentatives. 
great time and change of -conditions has been the fixing into 
race groups of the original variable characters of the species. ° 
especies, at its first appearance in the Silurian presented 
a decidedly new combination of characters for the genus and 
