NO. II ECHINODERMS CLARK 13 



Description. — The test is 17 mm. in horizontal diameter, and 8 mm. 

 in height ; the apical system and the peristome are both 8 mm. in 

 diameter ; the primary spines are 22 mm. long. 



The test is circular, much flattened, twice as broad as high, with 

 strongly curved sides ; the apical system is flat ; about the peristome 

 the border of the test is slightly incurved ; the apical system and the 

 peristome are of the same size, their diameter being one-half that of 

 the test. 



The ambulacra at the ambitus are 2.5 mm. broad, and the interam- 

 bulacra are 8 mm. broad, the width of the ambulacra being 31 percent 

 of that of the interambulacra. Above the ambitus the ambulacra are 

 slightly sinuate. 



The interporiferous zone of the ambulacra is elevated slightly above 

 the surface of the poriferous zones on either side, which slope upward 

 toward it. At the ambitus the interporiferous zone is 1.2 mm. in width 

 and the adjacent poriferous zones are 0.6 mm., the interporiferous 

 zone being about twice the width of the adjacent poriferous zones. 



In the interporiferous zone the marginal series of tubercles are very 

 regular. The tubercles in these series are large, their diameter being 

 markedly greater than that of the adjacent pores, and they are sepa- 

 rated from each other by about their own diameter. Between the 2 

 rows of marginal tubercles are 2 rows of smaller tubercles. The 

 tubercles in these 2 inner rows are about as large as a pore. They are 

 situated in the lower angles of the plates, and their outer side is 

 tangent to the midambulacral articular lines between the plates so 

 that the interporiferous zone shows no naked median area. The tuber- 

 cles of these 2 rows alternate with each other, and also with the large 

 tubercles of the marginal rows. 



In the interambulacra the areoles are low, and their edges are not 

 raised. Those at the ambitus are slightly oval transversely, and those 

 below become progressively more and more strongly oval. The 

 areoles at the ambitus measure 3.6 by 3 mm. The 2 or 3 areoles 

 nearest the peristome are confluent, the 2 uppermost are separated 

 by 2 rows of tubercles, and the second and third from the apical 

 system are separated by 2 rows of reduced tubercles, or are in some 

 cases confluent ; the areoles at the ambitus are separated from those 

 above and below by usually a single row of tubercles. The boss is 

 rather low with a small mamelon having a small perforation, which 

 shows no trace of crenulation. The edge of the platform of the areoles 

 below the ambitus is smooth ; at the ambitus it bears from 20 to 25 

 tubercles, though the row may be more or less deficient. Around each 



