BRACHIOPODA. 
67 
gate scars, which seem to correspond to the laterals of the ventral valve, are 
here altogether in the upper half of the shell, diverging widely in their exten¬ 
sion forward, and are generally very slightly impressed. In the cavity of the 
valve there is a low, rounded median ridge, which extends from a point near 
the hinge-line forward to a little below the mid-length of the valve. About 
the middle of the shell there are two small scars. These are usually striated 
longitudinally, the median ridge passing between them. The area is coarsely 
striated. 
Type, Obolella chromatica, Billings. 
Observations. Since the date of Mr. Billings’ studies of Obolella,* * * § much 
valuable information in regard to the character of the interiors of these shells, 
especially of the brachial valve, has been contributed by Mr. S. W. Ford, in 
various papers in the American Journal of Science, and by Mr. C. D. Walcott, 
in Bulletin No. 30 of the United States Geological Survey. Mr. Billings 
assumed three species as types of his genus: (1) 0. chromatica ; (2) the Avicula? 
desquamata, Hall (= 0. crassa, Hall) from the limestones near Troy, N. Y.; 
(3) “A small species from the Potsdam sandstone of the St. Croix River,” which 
had been described by Professor Hall as Lingula ? polita, f and was subse¬ 
quently:]: referred with some degree of doubt to the genus Obolella. 
The original figures of O. chromatica , the first- 
mentioned type-species, gave but a very imperfect 
representation of the character of the interior of the 
pedicle-valve, and showed only the two elongate 
lateral muscular scars.§ Subsequently the figures 
Obolella chromatica. 
After Billings. 
Fig. 28. Interior of pedicle-valve. 
Fig. 29. Interior of brachial valve. 
, . , TTiiinTTv Fig. 29. Interior ot brachial valve. 
here copied, were published by Mr. Billings m the 
American Journal of Science,|| giving the interiors of both valves, as far 
as then known to him; previously, however, in the same journal,U he 
* Palaeozoic Fossils, vol. i, 1861; Canadian Naturalist, 1872; Amer. Jour. Science, 1872; Idem, 1876, etc. 
f 1860. Report Geological Survey Wisconsin. 
I 1868. Sixteenth Report N. Y. State Cab. Nat. Hist. 
§ Palaeozoic Fossils, vol. i, p. 7. 
i Vol. xi, 1876, p.,176. 
IT Vol. iii, 1872. 
