444 University of Kansas Geological Survey. 



per side showing irregular lines ( probably lines of segmenta- 

 tion) , as of overlapping concentric plates ; surface in some 

 specimens rugose, in others smooth. 



This species differs from Serpula tenuicarinata in not having a 

 carina, having nearly twice the transverse diameter, being more 

 cylindrical, and (possibly) having more than twice the length. 



Specimens of Serpula plana are very abundant in the upper 

 horizon of the Ostrea shales of the Fort Benton group. Speci- 

 mens have been collected by the writer from the above-named 

 beds, on the Smoky Hill river, in Ellis and Trego counties ; 

 north of Hays City, on the Saline river ; and at the southern 

 point of the Blue Hills, in Mitchell county. 



MOLLUSCA. 



Ostrea congesta Conrad. Plate xcix, figs. 10, 11, 13. 



Ostrea. congesta Conrad, 1843, Nicollet's Rep. of Explor. in the Northwest, 

 p. 167; Hall, 1856, Pac. R.. R. Reps., vol. in, p. 100, pi. i, f. 11 ; Meek, 1876, 

 U. S. Geol. Surv., vol. ix. p. 13, pi. ix, ff. la-f ; White, 1880, 4th Ann. Rep. 

 U. S. Geol. Surv., vol. ix, p. 294, pi. xxxix, ff. 11, 12, 13; Stanton, 1892, 

 Bull. U. S. Geol. Surv., p. 55, pi. i, ff. 5 and 6. 



Meek's description: "Shell elongated; upper valve flat; 

 lower valve ventricose, irregular ; umbo truncated by a mark 

 of adhesion. — Conrad. 



"This is a small, thin shell, the individuals of which are 

 often crowded together in considerable numbers, so as to as- 

 sume quite irregular forms. In cases where the individuals 

 had room to grow without interruption, the young shell is 

 usually found to be of an ovate form, and attached by the 

 whole under surface of the lower valve, the beak of which is 

 pointed, provided with a small triangular area, and usually 

 turned a little to the left. In this form they continue to grow to 

 lengths varying from twenty-five hundredths to one inch, when 

 the margins are abruptly deflected upward at right angles to 

 the flat attached base, and produced in this direction often for 

 as much as an inch or more ; the greatest extension being on 

 the lateral margins and at the extremity, opposite the beaks. 

 When seen at this stage of their growth, separated from the 

 body to which they were originally attached, and lying partly 



