286 Field Columbian Museum — Geology, Vol. II. 



The basal plates are three in number. Together they form an ob- 

 conical cup comprising about one-third the height of the calyx. Two 

 are quadrangular and equal in size, the other is pentangular and larger. 

 The proximal end of each plate is provided with a decided median 

 ridge, giving a triangular base to the cup in which is a circular 

 depression for the reception of the stem. The radial plates are four, 

 about equal in size, three pentangular and one quadrangular; the 

 distal margins are thickened and beveled towards the interior. The 

 beveled edge of each plate is pierced by pores, the exact number of 

 which cannot be determined from this specimen. 



This species is founded on a single silicified specimen (Mus. No. 

 P 8415), having the dorsal cup complete and in a good state of preser- 

 vation, but the dome is missing. In form it appears to occupy a 

 position between that of Z. howardi and Z. globosus. The descrip- 

 tion of Z. howardi* is as follows: "Body subovate or pear-shaped, 

 greatest diameter at the upper third, pointed below, length more 

 than twice the diameter; base subhexahedral." Z. pyriformis is 

 distinguished from Z. howardi by being much shorter in proportion to 

 its greatest diameter and by the form of the base ; also the opening 

 formed by the distal edges of the radial plates is much larger. From 

 Z. globosus, which it resembles in general proportions, it differs in the 

 form of the base and general outline of the calyx. 



The comparative characters of the three species may be shown in 

 tabular form as follows: 



Ratio of F m f b e Form of Position of 



diameter to height or as basal cup greatest diameter 



Z. globosus 6:7 Hemispherical Wider than Near middle 



high 



Z. pyriformis 3 :4 Short Wider than Near middle 



triangular high 



Z. howardi 1 '.2 Long Higher than Upper third 



hexagonal wide 



Locality: The type specimen (Mus. No. P 8415) was collected by 

 the writer in the clay pockets in the Niagaran limestone at Romeo, 

 Illinois. 



Examination of a large series of specimens of Z. howardi from St. 

 Paul, Indiana, showed two individuals of Z. pyriformis. 



* 17th Rept. Geol. Surv. Indiana, p. 643. 



