76 CONTKXBUTIONS TO THE GEOLOGY OF MAINE. [bull. 165. 
mens is well represented by Hall's figs. 6 a, o, and c of PL XXXIV, 1 
which by Hall are referred to Conrad's species R. semiplicataf. One 
of the specimens has the smooth condition of surface near the beak 
described by Conrad as characteristic of R. semiplicata, but examina- 
tion of our specimens makes it evident that this is here due to exfoliation, 
and that specimens, when well preserved, have evident plications as 
near to the beak as they can be distinguished for smallness of size. 
Fig. 3 of PL XXXV well represents another specimen. This is called 
"young of R. formosa" by Hall. 
The forms described and figured by Billings as R. asjpasia, excluding 
those he regarded as old specimens, should be taken as the type of his 
species. This will leave out certain forms, of as small size as the 
others, in which the sinus has two or three, and the fold three or four, 
plications. This latter form the writer takes to be a simple variety, 
but as it is a distinct form it confuses the definition of Billings's 
species to leave it among the other small forms, since only old 
specimens were supposed to have this character by Billings, whereas 
study of the specimens makes it evident that from the earliest stage of 
growth at which the surface plications are visible to the front of the 
shells no increase in number of plications occurs in any of this group 
of rhynchonellas. 
The use of Billings's name, although the same forms were previously 
recognized and figured by Hall, may be defended on the ground that 
Billings clearly distinguished, defined, and figured the diagnostic 
characters of the form, while Hall distributed the same form under 
more than one species and confused it with others of different form. 
Having thus recognized Billings's species R. aspasia and its typical 
characteristics, it should be noticed that there are specimens in the 
Square Lake fauna presenting the variant characters represented by 
Hall's figs. 12, 14 £, of PL XXXI V, referred by Hall to R. transversa?, 
but differing from 6 a, h, c, of the same plate by the possession of 
two more plications in the fold and in the sinus, the plications on the 
side being finer and more distinct. The writer has recognized this as 
R. transversa Hall, selecting as typical, from the several forms included 
by Hall under that name, one which seems to constitute the median 
combination of characters. It would be altogether improbable that any 
two paleontologists would distribute the original specimens figured by 
Hall under the names R. inutilis, R. transversa, R. rudis, R. jplano- 
convexa, R, sulciplicata, and the Delthyris Shaly limestone specimens 
of R. formosa in the same groups under which they are originally 
described, or under any groups that would be described as specifically 
distinct by another person. By this remark it is not intended to express 
the opinion that there may not be distinct specific groups developed 
among the forms described under these names, but certainly it is 
iPal. New York, Vol. III. 
