Strophomena and Other Fossils 
87 
Plectambonites sericea has a considerable range in the Liberty 
bed, not only in Wayne county, in Indiana, but also in Preble 
county, Ohio, and in the adjacent counties, and must be used 
with care in determining the base of the Liberty. It is the second 
great influx of Hebertella insculpta, followed this time in close 
succession by Dinorthis subquadrata, which marks the base of the 
Liberty. 
Strophomena planumbona-gerontica, var. nov. 
{Plate IV, Figs. 2 A, B, C ; Plate XI, Fig. 6) 
One of the extreme variations of Strophomena planumbona is 
remarkable for its decidedly gerontic appearance. The shells 
are rather small and very thick. The brachial valves are strongly 
convex. The hinge-area of the pedicel valve is distinctly higher 
along its entire length than in ordinary specimens of this species. 
The general outline of the shell is semicircular, or subquadrate. 
The length usually varies between 26 and 33 mm. The ratio 
of the length to the width along the hinge-line varies from 0.87 
to 0.68. The ratio between the depth or greatest convexity of 
the shell and its length usually varies between 0.50 and 0.58. 
The gerontic character frequently is indicated by numerous lines 
of growth near the anterior margin of the shell, the shell having 
increased greatly in thickness without any proportionate increase 
in length. 
Shells of this description occur in the Blanchester division of 
the Waynesville bed, near Clarksville, and Oregonia, Ohio, and 
near Roseburg, at Madison, and at the mouth of Bull Creek, in 
Indiana. A similar individual was found near the top of the 
bluff directly west of the wharf at Gore Bay, on Alanitoulin 
Island (plate XI, Fig. 6). 
Strophomena rugosa, Blainville 
The following description of Strophomena rugosa was published 
by H. M. Ducrotay de Blainville in his Manuel de Malacologie 
et de Conchiologie, page 513, in 1825: 
Animal tout-a-fait inconnu. 
Coquille reguliere symetrique, equilaterale, sub-equivalve ; une valve 
plate et Fautre un peu excavee; articulation droite, transverse, offrant 
