89 Buoa Limestone, Whitney 



peristome, are small granules. Smaller granules also encircle 

 the plates. The pores are round, uniserial, and a little irregular 

 in the region of the peristome. 



The interambulacral areas are broader than the ambulacral 

 being 4 mm. wide in the placogenous zone, 7 mm. in the median 

 zone, and 3.75 mm. at the edge of the peristome. They consist 

 of two rows of large, alternating plates, five in each row. The 

 primary tubercles decrease gradually in size from the abactinal- 

 surface to the peristome where they are about as large as the 

 tubercles of the ambulacra area. On each plate there is a promi- 

 nent, crenulated boss bearing an imperforate mamelon sur- 

 rounded by a large areola. Mammillated granules are so arranged 

 around the margins of the plates that there are two on the upper 

 adradial corners, while on the median margins of the plates there 

 are from three to five. This is not an invariable distribution, for 

 in the specimens studied, there is some change from area to area. 

 There are miliaries between the tubercles of the adjoining plates 

 along the median line. These are also found between the granules 

 bordering the plates. 



The peristome is large and circular. The basicoronal plates 

 bear ten moderately deep branchial incisions. The interambul- 

 acral areas are straight between the incisions, and slightly broader 

 than the ambulacral lips which are bilobate. 



The periproct is elevated, subeliptical, the posterior side being 

 nearly straight; the anterior, subangular. 



The apical apparatus is large, circular, and slightly concave 

 near the outer margins. It is composed of five ocular and five 

 genital plates and one supernumerary plate, the suranal, which 

 is at the center of the system. The right anterior genital plate 

 has an incision of irregular form. 



This species differs from 5. texana Roemer in that it is not 

 so elevated and has fewer ambulacral and interambulacral plates. 

 The character of the interambulacral lips is also different, those 

 of S. texana having two lobes whereas those in 5. volana are 

 straight. The periproct is also different in form. This species 

 resembles 5. mexicana Schliiter, but it is more depressed and has 



