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, Oholella chromatica, Billings. 



OiiSERVATiONS. 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 

 assnmed three species as types of his genua: (1) 0. chromatica ; (2) the Avicula? 

 desquamate. Hall (= O. 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- 

 quently4: referred with some degree of doubt to the genus Obolella. 



The original figures of 0. 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 oboieiia chromatica. 



After Billings. 



lateral muscular scars. § Subsequently tlie tigures no. 33. interior or pedicie-vaive. 



• ,1 Fig. 29. Interior of 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,!! he 



* Paleozoic Fossils, vol. i, 1861 ; Canadian Naturalist, 1S72 ; Amer. Jour. Science, 1872 ; Idein, 1876, etc. 



t 1860. Report Geological Survey Wisconsin. 



I 1863. Sixteenth Report N. Y. State Cab. Nat. Hist. 



§ Palaeozoic Fossils, vol. i, p. 7. 



i Vol. xi, 1876, p. 176. 



l[Vol. iii, 1872. 



