BRACHIOPODA. 
239 
the valves; usually, however, the septa of the brachial valve are very short 
and rest upon the inner surface of the shell. It sometimes happens that 
these septa unite before reaching the inner surface, and the spondylium 
thus formed is supported by a very low axial septum. This is the case in the 
original specimen of P. bisinuatus, McChesney, and in the Wisconsin shell refer¬ 
red to that variety by Whitfield.* It is more conspicuously developed in the 
Iowa shell which, has just been mentioned as P. oblongus, var. subrectus, and it 
serves to confirm the varietal character of that form. It has been already 
observed that the union or independence of these dorsal septa in the genera 
Anastrophia and Parastrophia can be regarded as a feature of only secondary 
importance. In the later pentameroids it will be found that the difference 
becomes fixed and of more positive significance, but in the Silurian shells it is 
still a variable feature, but not of usual occurrence. At times an exceedingly 
faint obsolescent radial plication of the exterior is observable in P. oblongus, and 
this feature is also occasionally apparent in P. pergibbosus. Hall and Whitfield, 
and more noticeable in P. occidentalis. Hall, of the Guelph fauna of the Province 
of Ontario, and of the Niagara fauna of Ohio and Wisconsin.! 
* Gedlogy of Wisconsin, vol. iv, p. 290, pi. xvii, tig-. 3. LS82. 
t The shells which are currently refei-red to Peidamerus 2 ^erffibbosus also vai-y not a little among- them¬ 
selves, and it would be no difficult matter to acquii-e a series of foi ms to demonstrate that this is l)ut another 
extreme of development which has originated in P. oblongus. The originals of P. 'pergibbosiis from the 
Niagara dolomites of Darke county, Ohio, ai-e rather small shells with long, oblique cardinal slopes, nar¬ 
row umbones and very deep valves. In the limestones about Milwaukee shells of this character attain 
great size, and in the chert of Jones county, Iowa, occui-s a very small shell which cannot be separated fi om 
this species by any decisive characters. 
Mr. Whitfield has tignred J as one of the variations of P. obhmgus, a gibbous shell from the upper 
coral beds of the Niagara gi-oup at Ashfoi-d, Wisconsin; a similar, though iiersistently smaller shell 
abounds in the dolomites of the Maquoketa region near Dubuque, and at Hopkinton, Iowa. The latter has 
been generally identified as P. pergibbosus. It is, however, quite a ilifferent shell from that occuring in 
Darke county, Ohio, its full, rotund valves, broad across the cardinal region, producing an expression dis¬ 
tinct from that of P. pergibbosus, while the suggestion of trilobation of the surface which is shown on all the 
specimens examined, indicates its neai-er relations to P. oblongus; as a cori-ected identification of this shell, 
the name P. oblongus, var. Maquoketa, is suggested. It is observed above that the variety of P. oblo7igus, 
prevailing in Ohio (var. bismuatus), is represented with extreme i-arily among the Clinton shells of New 
York. Similarly, the variety Maquoketa is known to occur on this side of the Mississippi only, in the Wis¬ 
consin locality cited. On Plate LXVIII, figure 13, there is given a figure of a shell of great size, probably 
from Indiana, which is nearei- to this than to any other foi-m of P. oblongus. 
i The differences between P. pergibbosus and the P. occidentalis, Hall, from the Guelph fauna are also 
obscure. The latter has the cardinal slopes very broad, the axial slopes flattened or depi-essed and the 
t Op. dt., pi. xvii, figs. 8, 9. 
