344 BULLETIN: MUSEUM OF COMPAKATIVE ZOOLOGY. 



spines, which are often borne on the abactinal system too ; when the spines are 

 large and conspicuous, they are called primaries ; smaller ones are called secon- 

 daries; the smallest are miliaries. In some flattened Echini the ambulacra on the 

 upper surface are laterally expanded in a conspicuous manner, and from their fan- 

 cied resemblance to a flower, each is called a. petal. On the flat, lower surface of 

 some species the ambulacra are indicated by furrows radiating from the mouth, 

 but the pores are exceedingly small and difficult to see. The furrows divide 

 on leaving the mouth and diverge steadily until near the margin of the test, when 

 they abruptly converge and disappear without meeting. These ambulacral fur- 

 rows are concealed by the spines and in most specimens can only be followed 

 on the bare test. 



Key to the Sea-Urchins of the Peruvian Coast. 



Anus vertically opposite mouth, which is at center of lower surface and 

 provided with jaws and teeth. 

 Periproct covered by 4 (rarely 5 or even more) approximately equal 

 plates. 

 Color black or deep purple ; numerous secondary spines present 



Tetrapygus niger 



General coloration reddish brown ; no secondary spines. 



Plates of abactinal system and adjoining bare interambulacral 



areas, so finely granular as to have an almost velvety appearance, 



prettily marked with deep red in contrast to gray or whitish 



ground color; usually no ocular plate (sometimes one) reaches 



periproct Arbacia stellata 



Plates of abactinal system and adjoining bare interambulacral areas, 

 coarselj' granular, not marked with deep red; one, two, or three 



ocular plates reach the periproct Arbacia spatuUgera 



Periproct covered by many small unequal plates. 

 Test (except the flattened lower side) covered by short, very stout, 

 truncate spines, forming a close, smooth pavement over the 



whole Podojthora pedi/era 



Test almost completely covered by unequal, more or less elongated 

 and pointed spines. 

 Test longer than wide, though often only slightly so ; color, dark 



purplish'or deep reddish brown Echinometra van brunti 



Test with circular outline ; color, more or less greenish. 

 Ambulacra with 4 (rarely 5 or 6) pairs of pores in each 



arc Strongylocentrotus gibbostis 



Ambulacra with 8-10 pairs of pores in each arc 



Strong ylocentrotus albus 

 Anus not vertically opposite mouth, the line connecting them being 

 approximately horizontal. 

 Test very flat with a deep notch in the margin of eacli ambula- 

 crum, or a broad slit (hmule) formed by the closing of this notch at 



