286 FiELD CoLUMBIAN Museum — GEo.ocy, Vot. 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. howard1* 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. howard1 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 Bhan nee Form of Position of 
diameter to height basal cup greatest diameter 
Z. globosus 6:7 Hemispherical Wider than Near middle 
high 
Z. pyriformis and Short Wider than Near middle 
triangular | high 
Z. howardi 12 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. howard from St. 
Paul, Indiana, showed two individuals of Z. pyriformis. 
¢ 
* 17th Rept. Geol. Surv. Indiana, p. 643. 
