What is the Olenellus Fauna? — Matthetv. 403 
just below it. Orthis is absent from tlie Protoleiius fauna un- 
less a single doubtful valve may indicate it. 
Kutorgina occurs in the Upper Paradoxides beds of Sweden 
and the Lower Paradoxides beds of New Brunswick, and is 
reported as occurring with 0. callavii in England. Acrothele 
ranges through the Paradoxides beds, also above and below 
them, and so giyes no help; and so of Lingulella and Obolella. 
Gathering the indications available from the genera of the 
Olenellus fauna of the original district in Vermont and its 
extension eastward, (the true habitat of the original Olenel- 
lus fauna) they seem to the writer to indicate that this fauna 
should be placed either at the summit of the Paradoxides 
beds; or be considered as a fauna living cotemporaneously 
with Paradoxides, but under different conditions of tempera- 
ture and other surroundings. Walcott calls it a littoral fauna, 
but so to some extent is the Paradoxides fauna, especially the 
earlier species. If the faunas were cotemporary then it would 
seem that a land barrier, or difference of temperature of the 
sea, must have kept them apart. The former conjecture can- 
not be sustained if the Attleboro (Mass.) fauna pertains to 
the Olenellus assemblage. 
There remains for consideration the Olenellus fauna of west 
America. Here we again meet a commingling of Middle 
Cambrian forms* with the genus Olenellus, so that we are left 
in doubt as to whether these are cotemporaiy with Paradoxi- 
des, or are later than it; but whatever be the horizon for 
Olenellus thonipsoni and its fauna, to this the western species 
will naturally conform. 
On the ground of the greater simplicity of the Scotch spe- 
cies, Peach is of the opinion that the genus Olenellus origi- 
nated in Europe and spread to America. Tliis view finds 
support on other grounds than he urges, viz: (1st) compara- 
tive size of the species on the two sides of the Atlantic, and 
(2d) the greater complexity of the American Olenellus fauna. 
It is a rule that holds good for various genera of trilobites, 
e. (/., Paradoxides, Dicellocephalus, Triarthrus, that the early 
species are small and as later ones arise they exhibit larger 
species which flourish for awhile and then suddenly disappear 
*Kutorgina, Orthis, Zacanthoides, Crepicephalus (and Oryctoceph- 
<ilus ?). 
