LAMELLIBRANCHIATA. 619 
Rhytimya sinuuta.] 
This genus is placed with much confidence into the same family as Hall’s two 
Devonian genera Pholadella and Cimitaria and the Carboniferous genus Allorisma, 
King. It is with the latter, however, that the implied relationship is easiest estab- 
lished. The general expression of the shells is not much unlike in the two genera, 
and in both the surface is grano-lineate and concentrically plicated; but here we 
find one of the peculiarities of the Lower Silurian genus. In the latter, namely, the 
folds are, when not entirely restricted to the anterior end, at any rate always the 
strongest there, while in Allorisma they are strongest in the umbonal and central 
parts of the valves. The hinge and the muscular impressions also, in the absence of 
any knowledge to the contrary, are believed to be very nearly the same in the two 
genera. The principal difference probably is the absence of a lanceolate escutcheon 
in Rhytimya. A well defined escutcheon is developed also in Pholadella and Cimi- 
taria and these genera are further distinguished from Rhytimya by their large 
umbones. 
In having the concentric surface markings strongest on the anterior end, these 
shells agree with Sedgwickia, McCoy, founded upon Carboniferous species. But after 
a careful comparison with the figures and descriptions of the species which McCoy 
himself placed under that genus, I am quite couvinced that the Lower Silurian 
types are not congeneric with the Carboniferous forms. There would be equally 
good reasons for including them in the same author’s genus Sanguinolites. 
With the exception of R. sinuata, which is from the middle Galena of Minnesota 
and next described, the genus is known only from the rocks of the Cincinnati group. 
The total number of species known is nine. Of these six are new and three have 
been described and referred to other genera, namely, 8. A. Miller described one under 
the name of Orthodesma byrnesi, and Whitfield two under the names of Orthodesma 
mickleboroughi and Sedgwickia lunulata.* The original of the last species has a well 
developed lunule and is much shorter than any of the other species. But it is evident 
that the specimen has been much distorted by pressure. Descriptions and figures of 
all the Cincinnati species except 2. lunulata are to be published in vol. vii of the 
reports of the Geological Survey of Ohio. 
RwYTIMYA SINUATA, 2. Sp. 
PLATE XXXVI, FIGS. 46 and 47. 
Shell rather small, about 25 mm. long, 12 mm. high at the beaks, and 11.2 mm. 
across the posterior end, with the thickness very nearly equalling the hight. Car- 
dinal outline declining anterior to the beaks, slightly sinuate posterior to them; 
*A recent examination of Billings’ originaltypes of Canadian Lower Silurian Lamellibranchas proves that his Cyrtodonta 
emma, from the Hudson River rocks of Anticosti, is really a species of Rhytimya. It is closely related to R, sinuata and 
R. producta, 
