OBOLIDtE. 375 



central muscle scars (h) varies considerably in the different specimens. Mickwitz's diagram- 

 matic figure shows the middle lateral scars (k) to be the smaller and situated close to the 

 median line. This, however, is not. the case in all specimens. In Plate VII, figure 9, the 

 middle lateral (k) is the larger and the two other scars are arranged quite differently from 

 those in Mickwitz's diagrammatic figure. In Plate VII, figure 5, the growth of the shell has 

 crowded all three of the muscle scars so that they are arranged in transverse lines. In Plate VII, 

 figure 4, they occur up on the side of the visceral cavity (v), the central scars (h) occupying the 

 greater portion of the space, the outside laterals (1) being crowded forward, and the middle 

 laterals (j), which are so large in figure 9, are scarcely to be determined at the umer angle in 

 figure 4. In Plate VII, figure 3, the points of attachment of the scars are on a ridge, and they 

 present no points of similarity to the position assigned in the diagrammatic figure of Mickwitz, 

 nor to the positions in Plate VII, figures 9, 4, and 5. The points of attachment are sunk deep 

 into the ridge, and on this account it is impracticable to distinguish between the central (h) and 

 middle lateral (k) scars. In figure 1 the scars are also on a high ridge. They are slightly 

 depressed and apparently range very much as in Plate VII, figure 5, except that the middle 

 laterals (k) are nearer the median line. It does not appear to be possible to distinguish the 

 centrals. 



Type. — 0. ajyoUinis Eichwald. 



Ohservations. — It is stated by Hall and Clarke [1892c, p. 339] that Mickwitz did not obtain 

 his results from the type specimens of Oholus apoUinis, but from a hitherto undescribed form, 

 Oholus quenstedti. Tins is true [see Mickwitz, 1890, p. 60, footnote], but in Ms final work 

 [1896, p. 128] Mickwitz diagnoses the genus and cites Oholus apoUinis as the type. He says 

 [1896, p. 25]: 



A more accurate study of the greatly increased material has convinced me that 0. quenstedti can not be maintained- 

 as a species, since it is merely the terminal link of a series of variations which, like var. maximus and ingricus, can be 

 traced back to 0. apoUinis. Thus, the typical species remains 0. apollinis Eichwald. 



Mickwitz [1896] has given in his exhaustive memoir on Oholus a very complete historical 

 sketch and fi.ill description of the genus and its subgenera so far as known to him. The study 

 was conducted with such care and thoroughness and the material was so well preserved that 

 our present knowledge of the adult shell of Oholus is nearly as complete as that of the adult 

 shell of the recent Lingula. The student is referred to the memoir of Mickwitz for the literature, 

 history, and geology of the Cambrian formations of the eastern Baltic region of Russia, for a 

 minute description of the external and internal characters of the shells of the Baltic species 

 of Oholus and of the relations of Oholus to Lingula and Oholella, and for detailed observations 

 on Oholus and its subgenera as known to him. 



Oholus and Lingula. — After studying the species from American rocks and a very good series 

 from the typical localities in Russia, I am not prepared to agree with Mickwitz that Oholus- 

 should be referred to the Lingulidse. In figure 34 the interior of the valves of Oholus and Lingula, 

 may be compared. 



