220 
Sandstone we have but a single example of the group, viz. O. 
musculosa, Hall. Iu the Corniferous Limestone, however, a 
second great expansion of the type occurs, and we find no less 
than six species of the group, viz. O. alsus, Hall, O. mitis. Hall, 
O. Livia, Billings, 0. Vanuxemi, Hall, O. Semele, Hall, and O. 
Cleobis, Hall; some of these being hardly separable from one 
another and from precedent forms. Lastly, in the Hamilton 
group, besides 0. Vanuxemi, continued from the Corniferous, 
we have four fresh representatives of the type, viz. O. leucosia, 
Hall, O. cyclas, Hall, O. Penelope, Hall, and O. idoneus, 
Hall. 
Or, again, we may take another series, which culminates in 
the well-known Orthis resupinata of the Carboniferous rocks. 
This series commences in a well-marked form with Orthis mul- 
tistriata, Hall, of the Lower Helderberg : it is represented in the 
Corniferous Limestone by the very similar O. propinqua, Hall 
(so similar as to be almost undistinguishable) ; it is continued 
in the Hamilton group by O. Tulliensis, Vanuxem, and O. 
Iowensis, Hall; whilst it is represented in the Portage and 
Chemung groups by O. impressa, Hall. 
Turning to the genus Strophomena, M r e find exactly the same 
phenomena. Thus, the large and important group of Stro- 
phomenoid shells typified iu the Lower Silurian by S. alternata, 
Conrad, and a number of allied forms, continues to be repre- 
sented in the Clinton by S. alternata, and, though without any 
conspicuous example in the Niagara group, is represented in 
the Lower Helderberg by the two well-marked forms, S. con- 
cava, Hall, and S. varistriata, Conrad. Later on, in the Corni- 
ferous and Hamilton groups, we find the type represented by 
a whole group of forms — S. inequistriata, Conrad, S. inequira- 
diata, Hall, S. Patersoni, Hall, S. textilis, Hall, and S. herni- 
spherica. Hall; which Hall considers as distinct species, but 
which Mr. Billings regards as probably nothing more than 
varieties of one protean form, which is continued into the 
Chemung group by S. Cayuta, Hall. 
Similarly, the S. Headleyana, Hall, S. punctulifera, Conrad, 
S. Leavenworthana, Hall, and S. cavumbona, Hall, all from 
the Lower Helderberg group, are hardly or not at all separable 
from the S. ampla of the Corniferous Limestone, a species which 
is also stated by Mr. Billings to occur in the intermediate 
formation of the Oriskany Sandstone. 
Again, the Strophomena patenta, Hail, of the Clinton group, 
related perhaps in turn to S. pectcn, Linn., of the older rocks, 
is represented in the Niagara formation by the nearly allied 
S. subplana, Conrad, which is followed in the Lower Holder- 
