148 Bulletin, Vanderbilt Marine Museum, Vol. IV 



an almost deformed appearance. Specimens attaining a length of 

 50 mm. have been authentically recorded. The "Ara" specimens 

 are about 57 mm. long diameter. 



The test is moderately thin, ovoid, its contour from above almost 

 elliptical, almost angular near the junction of the interambulacral 

 plates. This angular outline is most evident in old specimens, the 

 younger forms tending to be more ovoid. The plates composing the 

 test are each somewhat conical, rising slightly towards one side and 

 tapering gently towards the margin; from their highest point the 

 tubercles radiate irregularly towards the margin. The anterior ambu- 

 lacrum has part of the posterior edge of its apical part projected over 

 the anterior edge, completely closing the upper portion of the anterior 

 lateral ambulacra. It then recedes again and thus causes the ambu- 

 lacral groove to appear to commence at a distance from the apex and 

 form a distinct angle with the longitudinal axis. The unusual anterior 

 adambulacrum is divided into two connected cavities, by the closing 

 up of the slightly sunken edge of the anterior groove. When examined 

 from above the projecting lips form two triangular spaces, closely 

 covered with the small tubercles, to the anterior edge of which the 

 peripetalous fasciole extends and thence passes on to the edge of the 

 lateral ambulacra. The anterior lateral ambulacra are decidedly 

 longer than the posterior. The vertex is situated slightly behind the 

 apical system ; the posterior extremity is truncated vertically, forming 

 a sharp angle with the actinal plastron which has a small, well- 

 developed keel immediately below the subanal fasciole. The anal 

 system is flush with the test, small, elliptical, margined by eight plates 

 surrounding the small, interior plates. The lateral fasciole is narrow, 

 widening slightly under the anal system ; that portion of the fasciole 

 adjacent to the deep groove is narrow and the connecting parts 

 between the petals are narrow. 



The spines are numerous, fine and short excepting those that meet 

 above and almost entirely conceal the sunken ambulacra. The inter- 

 ambulacral plastron on the lower surface also is covered by longer 

 spines, which when worn become spatulate at the tip. The spines 

 also are somewhat longer on the upper lateral part of the posterior 

 ambulacra particularly towards the mouth, and also on the side of 

 the ambitus. 



References: Spatangus atropus Lamarck, Anim. sans Vert., Ill, p. 

 32, 1816, ibid, 12th ed., Ill, p. 327. 



