LAMBLLIBBANCHIATA. 619 



Khytimya sanuata.] 



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 Bhytimya. , A well defined escutcheon is developed also in Pholadella and Cimi- 

 taria and these genera are further distinguished from Bhytimya 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 convinced 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 B. 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, S. 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 B. lunulata are to be published in vol. vii of the 

 reports of the Geological Survey of Ohio. 



Rhytimya sinuata, n. sp. 



PLATE XXXVI, FIGS. « 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 Lamellibranohas proves that his Oyrtodonta 

 emma, from the Hudson River rocks of Anticosti, is really a species of Bhytimya. It is closely related to B. sinuata and 

 B. producta. 



