DESCRIPTION OF OLIGOPORUS COREYL 187 



28 and 29) agrees with the description of Oligoporus coreyi in the follow- 

 ing characters : Body small, globose, moderately thick plates ; interam- 

 bulacra twice as broad as ambulacra ; ambulacra composed of 4 distinct 

 columns of plates ; interambulacra in the middle composed of 6 columns 

 of plates ; pores of the ambulacral plates situated near the outer edge. 

 The pores are near the center of each half-area in plate 6, figure 25, but 

 that is because the specimen is viewed from the inner or proximal side 

 of the test and the pores pass toward the center of the half-area in trav- 

 ersing the thickness of the plate (see OUgoponts, plate 9, figure 50 ; see 

 page 184). The height of Meek and Worthen's type was 1.65 inches ; 

 the specimen here described, as far as preserved (plate 6, figure 28), 

 measures 1.87 inches in height. If perfect dorsally, it would probably add 

 at least half an inch to this measurement. If the type was a reasonably 

 entire specimen, which is not stated, our specimen would probably be a 

 little longer in the dorso-ventral axis. The type is stated as being about 

 two inches in breadth and the interambulacra are stated as twice the 

 width of the ambulacra. The authors do not say what the width of the 

 ambulacra and interambulacra is, but we can estimate these areas ap- 

 proximately from their measurement of the breadth. If the breadth, 

 which is two inches, represents fairly the diameter, as may be assumed, 

 then the circumference may be attained by multiplying the diameter, 2 

 inches, by 3.14, which gives 6.28 inches, or 15.8 centimeters, for the cir- 

 cumference. As the ambulacra are half the width of the interambulacra, 

 therefore the width of the ambulacra must be one-fifteenth of the circum- 

 ference, or 1.05 centimeters ; the width of the interambulacra would be 

 twice that amount, or 2.1 centimeters. Comparing the specimen with 

 this ideal measurement, I find that the ambulacra measure at the widest 

 part 1 centimeter and the interambulacra 1.9 centimeters. These meas- 

 urements, taken with the number of columns of plates, the inferred size 

 of the plates, etcetera, render it entirely probable that the specimen is 

 Oligoporus coreyi. In the type the surface is described as unknown, but 

 in this specimen the plates of the interambulacrum are thickly covered 

 with small bosses for the attachment of spines (plate 6, figure 29). No 

 spines are preserved. 



The specimen of Oligoporus coreyi here described is labeled as from 

 Indiana, but the label does not give a detailed locality or geological hori- 

 zon. The specimen is composed of thoroughly crystallized calcic car- 

 bonate, which is stained reddish brown with oxide of iron. It difi'ers 

 lithologically from Crawfordsville material and cannot, therefore, be 

 ascribed to that locality. The type was from the Keokuk group, and hav- 

 ing no evidence to the contrary this specimen is provisionally ascribed 

 to the same horizon. 



XXV— Bull. Geol. Soc. Am., Vol. 7, 1895. 



