PURPLE HEART-URCHIN. 183 



Echinus purpureas, Gmelin, p. 3197. 



Echinus lacuitosus, Pennant, Brit. Zool. IV. p. 69, t. xxxv. f. 76. 



Spatangus merhiiomdis, Risso, t. v. p. 280. 



The Heart-Urchins form a very natural division of the 

 Echinidte. Their name expresses their form ; their colours 

 are various shades of red, purple, and yellow ; most of 

 them bury themselves in sand or mud, and the greater 

 number of their spines are directed backwards, doubtless 

 for the facilitating of this custom of living interment. 

 The two apertures of their digestive canal are placed very 

 differently from their positions in the true Egg-Urchins, 

 the anal being terminal, and the oral eccentric and be- 

 neath. The mouth is protected by a projecting plate. In 

 separating the species referred to the old genus Spatangus, 

 I have taken a similar view of their generic distinctions 

 with that maintained by M. Desmoulins, and have not 

 adopted those proposed by Professor Agassiz, which seem 

 to me insufficient, and founded on the imperfect evidences 

 afforded by fossil species. The characters proposed by the 

 distinguished Swiss zoologist for the separation of his 

 genera, I must regard as appertaining rather to family and 

 species. Those afforded by the marks or impressions seen 

 on the back, round the anus, and below the anus (in the 

 dried specimen generally naked, in living examples thickly 

 covered with short slender spines), are constant, certain 

 arrangements of them agreeing with certain arrangements 

 of the ambulacra in each generic assemblage of forms. 

 The five British Spatangi which I find on our shores may 

 thus be well arranged under three genera, for which I have 

 severally retained the names of Spatangus, Brissus, and 

 Amphidotus, in order that synonyms be multiplied no more 

 than is absolutely necessary. The English terms of Bank- 

 Urchin, Mud-Urchin, and Sand-Urchin might be respect- 



