288 PROCEEDINGS OF THE ACADEMY OF [1889. 



spire depressed so as to be on a level with the outer turn, thus differ- 

 ing from S. roberti White, in which the spire is depressed considerably 

 below the upper surface of the last whorl, making the superior 

 carina very conspicuous. S. roberti appears to be the genetic successor 

 of S. lotus, but there is nothing to indicate that either species was 

 immediately derived from any of the Kinderhook forms, which are 

 all characterized by regularly rounded volutions. 



In the absence, from the Burlington beds, of the other generic 

 groups of gasteropods represented in the Kinderhook no further 

 comparisons of the forms from the two horizons can be made. How- 

 ever, as previously intimated, the general aspect of the molluscan 

 remains under consideration, from the lower division, points to the 

 existence over the region of a shallow secluded expanse of water 

 perhaps removed for the greater portion of the time from the im- 

 mediate influence of the great mediterranean sea that at this period 

 stretched away to the southward. 



I. Species from the Burlington Beds. 

 Platyceras cyrtolites McChesney. 



Platyceras cyrtolites McChesney, 1860. Desc. New Pake. Foss., p. 

 71. 



This form was originally described from the Burlington limestone 

 of Calhoun county, Illinois. It has recently been obtained from the 

 upper layers of the same horizon at Burlington, Iowa. At the latter 

 place it is quite rare ; and the shell is usually exfoliated. It appears 

 to have its nearest affinities in certain forms of the superimposing 

 Keokuk beds. 



Platyceras equilaterum Hall. 



Platyceras equilaterum Hall, 1859. Geol. Iowa, vol. I, Supp., p. 

 89. 



Platyceras equilaterum Meek and Worthen, 1873. Geol. Sur. 

 Illinois, vol. V, p. 518. 



Platijceras equilaterum Keves, 1888. Proc. Am. Philosophical 

 Soc, vol. XXV, p. 236. 



This species is one of the most characteristic gasteropods of the 

 Keokuk limestone. In the upper Burlington rocks it has lately 

 been found attached to the ventral surface of Gi/bertsocritius typus 

 (Hall). The sedentary habits of the members of this genus have 

 been fully discussed elsewhere and need not he considered here. 

 Suffice it, that fifteen species of this group have been observed on the 

 calyces of various crinoid-, particularly those having a more or 



