184 SPATANGACE.E. 



ively applied to each of these genera as descriptive of 

 locality. The organic importance of the impressions on 

 which I have mainly founded my characters is as yet un- 

 discovered ; though from the circumstance of the genus 

 Spatangus (proper) having no such marks on the hack, 

 while they are very regular and evident in the two other 

 genera, and largest in that inhabiting thick mud, may we 

 not conjecture that the little spines clothing those impres- 

 sions serve the animal for progression through the medium 

 in which it buries itself? When we look through a mag- 

 nifier at the surface of one of these impressions when the 

 spines are rubbed away, we see the minute tubercles 

 which cover it are very different from those bearing the 

 true spines ; and doubtless the purpose of each set of 

 spines is different also. Strange to say, these impres- 

 sions, though affording excellent and most obvious spe- 

 cific as well as generic characters, have escaped mention 

 in the descriptions of most of the species. The attention 

 of describers of Spatangi has been chiefly directed to the 

 number of ambulacra, whether four or five, and to the 

 depth of the dorsal grooves ; most fallacious characters, 

 especially the former, as in all the species there are four 

 perfect and lateral dorsal ambulacra, and one imperfect 

 and central ; which latter, however, has frequently through 

 carelessness escaped notice, the holes being sometimes 

 obsolete without though evident within. 



The Purple Heart-Urchin is one of the handsomest of 

 the British Echinidse, and by much the largest and most 

 elegant of our native Spatangi. It is the only native 

 species of the restricted genus Spatangus. It grows to 

 the length of four inches by three and a half broad, and is 

 occasionally larger. It is of a deep purple colour with 

 pale spines. Some of these spines are very long and 



