H. S. Williams—Inje History of Spirifer levis. 457 
served in making comparison with other forms. After point- 
ing out the specific characters of S. levis I give the results of 
a minute comparison of them _ those of 8. fimbriatus of the 
Hamilton and earlier formatio 
This comparison has left little doubt that a genetic relation- 
ship exists between S. devis and the earlier form mbriatus. 
en an examination was made of the relationship of several 
species of the preceding geological formations to the typical 
form as characteristic of each of the former species, and thus 
representatives of the type were tli in each of the fos- 
oe formations back to the ar 
the Niagara group was found the earliest trace of the 
sodibthatien of characters found to be essential in S. /evis and 
fimbriatus and the other representatives seen in the interme- 
diate formations. 
The species which appears to be the central type of the orig- 
inal primitive species is Spirijer ha Hisinger, of the Niag- 
ara formation, of which I presume S. bicostatus H. may be re- 
garded but an extreme variety, ae S. sulcatus His. (at least in 
Pe art as referred by Hall), the extreme variety on the other side. 
he peculiarities of this species (S. craspus His.) are very tee 
abundance and wide distribution in the formation in which 
first appears. It being a characteristic species of the lip 
ac hie represented, in England and at several localities in 
rope. 
Where it does appear, it also assumes great variation of charac- 
ters, so that the three species in America, S. crispus, S. bicostatus 
and sulcatus, while good species in small collections, are rec- 
ognized, even by Hall as bordering on each other in some of 
their varietal forms. 
Also the three species ig a by Davidson in Great 
Britain (s sulcatus, S. elevatus Dalman and S. ertspus), corres- 
ponding in the main to a extreme forms identified by Hall 
in this country, are regarded by Davidson as doubtfully dis- 
tinct species (see Brit. ‘Sil. Brach., pp. 91 to 98) on account - 
the variations and intermediate forms. I have also traced o 
as well as the limited material at hand eoaid: allow, oe 
relationship to S. glaber Martin and gig age) acta forms. 
_ The study of the facts has led me to the following conelu- 
sions. Whatever theoretical aeocaon: we may give Z species, 
here are, in the first place, an abundance of individual organ- 
isms whose ‘remains are found in the Upper Silurian rocks of 
Kurope, Great Britain and America, presenting a few clearly 
marked distinctive characters, which are found v alaitps de- 
veloped in the individual forms, but so sae | in the various 
varieties as to cause careful naturalists to associate them as 
varieties of a single species. There are well marked typical 
