PALAEONTOLOGY OF IOWA. 487 



Geological formation and locality. In calcareous shales of the age of the 

 Hamilton group : Rock-island (Illinois), opposite Davenport (Iowa). 



Oithis vanuxemi. 



PLATE II. Fio. 2 a - i, 3 a -/. 



Orthis vanuxemi : HALL, Report of the Regents of the University on the N.Y. State Col- 

 lections of Natural History for 1856, p. 95. 



SHELL depressed orbicular, varying from circular to trans- 

 versely broad oval, and rarely a little longer than wide, 

 resupinate. Ventral valve most convex near the beak, gra- 

 dually depressed below and flattened towards the front ; beak 

 elevated, scarcely incurved. Dorsal valve most convex a 

 little above the centre, an undefined depression down the 

 middle, which expands into a broad flattened space, some- 

 times giving the base a straight appearance ; beak scarcely 

 denned : length of area about one-third the width of the 

 shell : foramen broad, partially filled with the strong car- 

 dinal process. 



SURFACE marked by fine closely arranged radiating tubular 

 stride, which increase by bifurcation and implantation, and 

 are crossed by very fine concentric lines, and towards the 

 margins by imbricating lines of growth ; striae with frequent 

 tubular openings upon the surface. 



Interior surface finely punctate. Interior of ventral valve 

 marked by a large foliate or flabelliform vascular impression, 

 and in older shells a strongly defined impression of the ad- 

 ductor muscles a little above the centre of this, and a pro- 

 minent cardinal process which is continued below in a ridge 

 along the centre of the valve. Margins crenulate with the 

 radiating strise, which are sometimes visible more than half 

 way to the beak of the shell. 



The specimens of this species from Rock-island are usually longer than wide : in 

 other localities, they are more nearly circular; and notwithstanding the variation 

 from the prevailing form of the New- York specimens, there appears to be no specific 

 difference. 



