40 G. F. MATTHEW ON TUB 



Prof. Jas. Hall had about this time, 1861, described the features of the interior of 

 Obolella polita, from which for the first time palaeontologists were enabled to gain a fair 

 knowledge of the peculiar internal markings of the shells of this genus, and it is evi- 

 dently congeneric with the species described by Billings. 



Messrs. Meek and Hayden also at this time described a species of Obolella from the 

 Black Hills of Dakota,' which Mr. "Walcott has since declared to be identical with O. polita. 

 To the three species first included in the genus Mr. Billings subsequently (1860) added 

 the following : " O. pretiosa from the Sillery sandstone, O. desiderata from the Levis grapto- 

 lite beds and O. Ida from the Levis limestone (Nos. 1 and 3) — the last three near Quebec.^ 



Subsequently (1876) Billings described more fully, from specimens exhibiting the 

 internal characters of O. chromalicn, the generic features of that particular species.* 



In 1872 Mr. Billings described two other species of Obolella from the conglomerate 

 limestones of the St. Lawrence Valley, which Mr. Walcott and others refer to a Cambrian 

 terrain, viz., O. gemma and 0. cîVfe.'' Of these the first is more linguloid in form than 

 any other species of the genus. The second, 0. circe, is described as presenting many 

 points of resemblance to O. crassa, but is distinct by the internal features of the ventral 

 valve. 



Meanwhile (in 1863) Mr. S. W. Ford had described another small species of Obolella 

 from the Primordial limestone of Troy, N.Y., viz., O. nilida!' Of this he obtained only 

 the dorsal valve, and the internal markings were not well enough shown in any of the 

 specimens to admit of description. 



Subsequently in 1881,' Mr. Ford wrote an excellent article on the genus in which he 

 compared at some length the specific characters of the species O. crassa, O. chromatica and 

 O. gemma. He also expressed the opinion that O. desiderata, Bill, and O. sagittalis, Salt, 

 did not belong to the genus, and would be found to constitute a new genus, which, how- 

 ever, he did not name 



In 1885 Mr. C. D. Walcott defined this genus — Linnarssonia — and gave for compari- 

 son figures of O. chromatica and of the species referred to the new genus. He also divided 

 the species left in Obolella into three groups, taking O. gemma and O. crassa as types of 

 the original genus.'' In the third group he included O. (?) polita Hall, and his own species 

 O. ambigua, but not defining this any further. 



The next important work on this genus was also done by Mr. Walcott, who figured 

 and described new examples of several of the species, and brought together the figures 

 and descriptions of others.' 



In these several articles and works are the data used for comparison of the characters 

 of the following species :— 



' Pal. Upp. Missouri, p. 4, figs. 3 a-d. 



■' Pal. Foss., p. C7. 



•' Of these species O.prclio.m i.s found to be a Linnnnmnia (a genus separated from Obolella by Mr. Walcott) 

 0. dcsi'dcrato is from OrJovician strata and of 0. /(?a the internal markings are unknown; none of those can be 

 counted as true Cambrian Obolelltu. 



* ' Am. Jour. Sci.,' Vol. IT., p. 17t). 

 5 'Can. Nat. d- Gaol.,' 1S71. 



6 ' Am. Jour. Sci.,' 3 Ser., Vol. V., p. 213. 

 ' ' Am. Jour. Sci.,' Vol. XXI, p. 131. 



* Billings, however, described 0. chromatica as the type of this genus. 

 3 ' U. S. Geol. Surv.,' Bull, 10 (1886). 



