ECHINUS MARGARITACEUS. 493 



Echinus margaritaceus 



! Echinus margaritaceus Lamk., 1816, An. s. Vert. 



As suggested by Lutken, this is a true Echinus in the restricted sense 

 (Desor, emend.), with very marked features. It is flattened, slightly conical, 

 resembling in outline E. elegans ; has, like E. norvegicus, a large anal system, 

 but, unlike either elegans or norvegicus, the genital plates are narrow, with 

 three small tubercles near the anal edge; the genital openings large. The 

 coronal plates are not high; the principal row of vertical intcrambulacral tu- 

 bercles is small ; the remainder of the plate is densely covered with minute 

 secondary tubercles carrying short, slender, yellowish spines closely crowded 

 together, which are a lower groundwork from which the primary spines, 

 long, slender, and white, project prominently. The rest of the space of 

 the plates is closely covered by miliaries, covering the whole test with its 

 huge pedicellariae, except the indistinct vertical median line, both in the 

 interambulacral and ambulacral region, which extends to the ambitus from 

 the pole. The mamelon of the tubercles is large, the boss very prominent 

 and well defined. The ambulacral area is broad, the principal tubercles 

 of the main vertical row but slightly smaller than those of the interambu- 

 lacral space, the secondaries covering equally closely the whole plate. 



Patagonia. 



Echinus melo 



I Echinus melo Lamk., 1816, An. s. "Vert. 



Almost globular, very ventricose. Actinal opening much smaller than in 

 E. acutus. Smaller, thinner, and less powerful auricles. Coronal plates 

 very elongate, so that, when specimens of the same size are compared to 

 E. acutus, we can at once discriminate them by the greenish-yellow tinge 

 of the test, while in acutus red and pink tints are most common. The 

 principal row of vertical primary tubercles is small ; the rest of the plate 

 is more fully packed with secondaries than is the case in acutus, and densely 

 covered with miliaries carrying globicephalous pedicellarios. Alternate 

 primary tubercles are frequently absent in both acutus and melo ; the 

 horizontal sutures are bare both in the ambulacral and interambulacral area, 

 forming a lozenge-shaped pattern of a lighter color than the main color of 

 the test. This feature is usually not so prominent in acutus, and fre- 

 quently totally wanting. In large specimens the secondary tubercles form 

 six or seven vertical rows of tubercles hardly extending beyond the ambitus. 



