SECONDARY AND TERTIARY FORMATIONS OF NORTH AMERICA. 139 



definite arrangement with relation to each other. Sometimes every cellule on a branch 

 will be found to be provided with one, at others, they are hardly as numerous as 

 shown in the figure. The ovarian vesicles are equally uncertain in their arrangement. 

 We frequently find branches without a single one, and on one specimen, nearly as 

 large as the one figured almost if not quite every cellule is provided with its ovicell. 

 These two appendages do not seem to have at all interfered with each other. Some- 

 times a cellule will be provided with both, at other times, neither is present. 



OLIGOTRESIUM, (K G.) G. and H. 



Colony discoidal, free ; convex above, concave below ; cellules on the convex or 

 upper surface ; arranged in radiating rows, with new rows occasionally interpolated. 

 New rows 'commence by an abortive cellule. Opening of moderate size. A certain 

 number of cellules are provided with a vibracular cellule, or '"'special pores" placed 

 laterally, between the rows cf ordinary cells. 



This genus differs from Limulites, in that a certain number of the cellules are pro- 

 vided with vibraculae. From Selenaria it is distinguished by the vibracular cell beinf 

 small and placed between the rows of ordinary cellules, while, in the latter genus, the 

 vibracular cell rejjlaces an ordinary one. 



0. vicksburgensis, G. and H., fig. 22. 



Lumdltes id., Con. Proc. Acad. Nat. Sci. Phila., vol. 3, p. 296. Id. Jour. Acad., 

 vol. 1, p. 127. 



Colony discoidal, of exceeding variable convexity, varying from almost flat to higher 

 than broad. Cellules on the upper or convex surface, arranged in radiating lines; in- 

 terpolated lines always commencing by an abortive cellule. Cellules subquadrate, 

 irregular in form and size, anterior margin convex, posterior, concave; always sep- 

 arated by an impressed line. Surface regularly concave. Mouth central to anterior, 

 never terminal ; large, oval to subquadrate, promixal margin usually straight. A 

 small raised lip sometimes encircles the mouth, occasionally only a portion of.it, and 

 at other times is entirely absent. 



There is a vibracular cell, usually oval or fusiform, with an opening of variable 

 shape, attached to each alternate cellule in the longitudinal lines, and placed at the 

 side or a little in advance, always in the line between the rows of cellules. This cell 

 is only placed with reference to the ordinary cellule to which it is attached, irrespec- 

 tive of the position of adjoining cellules in the neighboring series. 



Under surface marked by irregular radiating depressed lines, not always continued 

 to the edge of the colony. Between these lines the surface is convex and coarsely 



granulate. 



From the " Upper Eocene," Vicksburg, Miss. 



36 



