222 THE AMERICAN MIDLAND NATURALIST 



different individuals. In every case the specimens of this species 

 are prominently marked, by heavy folds on the upper part of the 

 first volution in older forms. The young specimens all occur in the 

 form of casts. 



"This species is related more or less closely to Naticopsis gigantea 

 of Hall and Whitfield, with which it is associated. Certain partially 

 crushed and distorted individuals of Naticopsis gigantea might 

 under certain circumstances be mistaken for this species. 



"Position and locality. The forms here described were secured 

 from the Middle Beds of the Hackberry Group of the Iowa Devon- 

 tan, at Rockford and Hackberry, Iowa; but it doubtless occurs 

 throughout the group at this horizon. Now in the author's col- 

 lection." 



This form was originally described as a variety of Naticopsis 

 gigantea Hall and Whitfield. (See 23d. Annual, N. Y. State Cabinet, 

 pp. 238-9, plate 12.) I am quite fully convinced that this form does 

 not belong to the true Naticopsis. Whether the generic differences 

 are so great as to place Floydia in separate generic rank is not yet 

 fully decided by some. For the present I do so, though it is true 

 that the two forms seem to intergrade. 



Mr. Webster wishes to correct the horizon as given for this 

 species. It also occurs in the Owen sub-stage. 



In the accompanying plate there are shown four specimens from 

 my own collection. Number one is an individual that has some of 

 the characteristics of both Floydia and Naticopsis; two is a small 

 specimen of Naticopsis gigantea Hall and Whitfield, and three is a 

 moderate-sized specimen of the same species. (888,889, an d 1 20, col- 

 lection of C. L. Fenton). Number four (No. 40, C. L. F.) is the 

 type of Floydia concentrica multisinuata of this paper, and number 

 five is a metetype of Floydia concentrica Webster (No. 95, C. L. F.) 



In the second specimen the extreme thickness of the shell will 

 be noted together with the three tiny folds near the outer lip. 

 The third specimen shows plainly the lines of growth and the 

 "strong, subregular, transverse folds or wrinkles" of the true 

 Naticopsis gigantea, and comparison with Floydia concentrica 

 shows them to be of very different character from those of the latter 

 species. Figure 5a shows a cross section of the shell of the specimen 

 illustrated in Figure 5. These figures will, I think, make more 

 plain than words the differences between Naticopsis and Floydia. 



