313 
[Foerste. 
Platystrophia regularise Slialer, is a form of Orthis biforata. Here 
again the mesial fold is occupied by three plications, and the sinus 
by two. 
It will be seen, therefore, that in the Clinton specimens which 
have actually come under my observation, an odd number of plica- 
ions usually occupies the mesial fold, and an even number the sinus. 
Now this is all reversed in the Lower Silurian forms at hand. 
Taking all the different varieties of Ortliis biforata now in the stu- 
dents’ collections at Harvard University I found that 128 speci- 
mens had an even number of plications on the mesial fold, and an 
odd number in the sinus, while only five showed the reverse ar- 
rangement. While it is almost absolutely certain that these dis- 
tinctions between Lower and Upper Silurian forms of this species 
will not prove constant even in American forms, still the fact, that 
they have borne all the tests I have been able to apply to them with 
a moderate degree of success, is worthy of mention. 
Spirifera radiata, Sowerby. 
(plate y, FIG. 6.) 
Spirifera radiata is figured by Hall from the Clinton Group of 
New York. It is of about the same size as the typical European 
forms ; the striae are usually a little finer. 
Spirifera tenuistriata , Slialer, from Anticosti is a somewhat 
smaller form with very much finer striae, eight and one-half in a 
width of 2 mm. This smaller form occurs also at Cumberland 
Gap, Tennessee, but the striae are here coarser, from six to seven, 
occupying a width of 2 mm. thus approaching a little towards the 
coarser striated Clinton forms of New York. In fact it seems im- 
possible to establish a species on the basis of finer striae although 
it might prove a convenient varietal designation. The Tennessee 
specimen is 11 mm. long, 15.5 mm. broad; the greatest width of 
mesial lobe is 4.5 mm. The mesial lobe is defined from the lat- 
eral lobes of the shell chiefly by shallow' but distinct grooves. The 
mesial lobe itself is low and gently rounded. Spirifera radiata is 
perhaps more correctly placed as a variety under Spirifera plica- 
tella , Linnaeus. 
Spirifera (Cyrtia) rostellum, Hall. 
(plate v, FIG. 5.) 
This species was first described from the Niagara of Louisville, 
