94 HAWAIIAN AND OTHER PACIFIC ECHINI. 



There is no group of Echini which shows such diversity in shape of the test 

 as the Spatangina. Some species are nearly spherical while some are greatly 

 flattened. In the Pourtalesiidae the test becomes extremely elongated, reaching 

 the limit in the almost tubular Echinosigra. As a rule the longitudinal axis 

 is longer than the transverse, while the latter exceeds the vertical. Ambulacrum 

 III is very often depressed below the level of the adjoining interambulacra and 

 thus the ambitus at the anterior end of the test is more or less distinctly notched ; 

 when the test is also depressed the outline is thus somewhat heart-shaped 

 whence the name " heart-urchin," sometimes applied to spatangoids. The 

 modification of the dorsal ends of the ambulacra to form "petals," as in the 

 Clypeastrina, is characteristic of many Spatangina, especially the more special- 

 ized forms of Amphisternata. But there are very many spatangoids in which 

 there is no indication of petals whatever. The presence or absence of petals, 

 their size, form, and completeness, their depression below adjoining inter- 

 ambulacra, and the relative length of the different ones are all matters of great 

 importance in classification. 



One of the most characteristic features of spatangoids is the narrow, 

 more or less linear, areas covered by very minute spines and known as fascioles. 

 These are lacking in most asternous and meridosternous forms but are present 

 in most amphisternous species. Unfortunately our terminology does not at 

 present distinguish between the band of minute spines and the band of minute 

 tubercles left when the spines are all rubbed off. Duncan would limit the 

 term to the band of tubercles but I have used the term to cover both this band 

 and the minute spinules borne thereon. The difficulty is not in practise a seri- 

 ous one, as it is easy to speak of "fasciolar spines" if one so desires. There 

 are six kinds of fascioles, the names of which are used in the following pages: — 

 marginal, peripetalous, internal, subanal, lateroanal, and anal. The marginal 

 fasciole is very seldom met with in Recent species; it lies at or just above the 

 ambitus and runs around the whole test. The peripetalous is very commonly 

 present; it lies on the dorsal side of the test and surrounds the area occupied 

 by the petals; it is never far from the tip of the petals but on the other hand the 

 petals never cross it; in some spatangoids with long petals, it may approach 

 so near the ambitus that it is hard to tell from a marginal fasciole. The internal 

 fasciole lies within the area of the petals and limits them at their proximal end; 

 it is found in but few genera. The subanal fasciole is found in many genera 

 of both meridosternous and amphisternous spatangoids; it surrounds a circular, 

 transversely ellipsoidal or oblong area below the periproct which is commonly 



