96 The American Geologist. August, i892 
Keokuk group, it is readily observed that this series of rocks 
forms the cuhninating period of a great crinoidal epoch; that its 
most ponderous forms occur at its summit, and its base contains 
the transition forms l)et\veen this and the Burlington group; 
there exists a gradual development from the base to the sum- 
mit. 
In 1878 in a paper on "Transition forms in Crinoids, " Proceed- 
ings Academy of Natural Sciences of Philadelphia, Messrs. 
Charles Wachsmuth and Frank Springer proposed the name of 
Crinoidal limestone for the Keokuk, and the Upper and Lower 
Burlington limestones, and clearly proved that these three lime- 
stones represent three successive grades of development; Froosts' 
genus Agaricocrintis appears in the Lower Burlington with two 
arms to each ray, attains its maximum in the Upper Burlington, 
and becomes extinct at the close of the Keokuk, when it reaches 
its extravagance of form, and has been found with four arms to 
all the rays. Eoemer's genus Bori/crinus appears in the Lower 
Burlington with a single spine on the apex of the vault, species 
small, and disappears in the closing of the Keokuk with its ex- 
traordinary feature of spines on spines, as represented by Dory- 
crimis gouldi. 
NEW LAMELLIBRANCHIATA. 
Plate VII. 
By E. O. Ulrich, Newport, Ky. 
No. 4. Descriptions of one new genus and eight new 
SPECIES. 
The greater part of the new forms that haA'e been determined 
during the progress of my work on the Lower Silurian Lamelli- 
branchiata of the northwest for volume iii of the final reports of 
the Geological and Natural History Surve}' of Minnesota, now 
going through the press, were recently published in the Nine- 
teenth Annual Report of that survey. In the last few months 
much new material has come into my hands, among it several 
species entirely new to me, and others that were too illy repre- 
sented in the original lot for description. In the present paper, 
and in another to follow, the more interesting of these additional 
new species are described, while the treatment of a few other 
