ECHINARACHNIUS MIRABILIS. 527 



posterior ambulacral areas. The edge of the test varies greatly in thickness; 

 it is more or less swollen and slightly convex on the abactinal side, or has 

 a thin edge, especially in younger specimens, which slopes gradually to the 

 apex. Apex (organic and geometric) eccentric anteriorly, particularly in 

 specimens where the posterior interambulacral space is pointed. In young 

 specimens the anterior part of the test is quite rounded, angular posteriorly ; 

 the broadest part of the test is then greater across the median posterior pair 

 of ambulacra ; with increasing size the outline becomes somewhat more cir- 

 cular, but the greatest breadth remains at the same point. The median 

 interambulacral areas are in old specimens slightly sunken ; this feature is 

 subject to great variations, as we frequently find specimens in which no trace 

 of this depression exists. The anus is placed on the edge of the test, rather 

 more on the upper side. The four genital openings are distant ; the pos- 

 terior pair somewhat farther apart than the anterior pair. The apical system 

 is comparatively larger than in E. parma. The ambulacral petals are of 

 uniform size, equally developed, extending about two thirds of the distance 

 from the apex to the periphery ; the poriferous zone is broad, two thirds as 

 broad as the enclosed ambulacral space ; the petals of the lateral ambulacra 

 are almost closed at the extremity ; the odd petal is usually more open at 

 the extremity than the lateral pair. The tuberculation of the abactinal part 

 of the test is close and uniform in size, slightly smaller in the median ambu- 

 lacral and interambulacral areas ; on the actinal surface the tubercles are 

 largest near the edge of the test, near the actinostome, and on the sides of 

 the ambulacral furrows, leaving a space of smaller ones between these two 

 points. On the lower side the ambulacral furrows begin to branch at a dis- 

 tance of about two thirds from the edge of the test, each branch forking 

 again somewhat nearer the edge, or sending one or two irregular smaller 

 branches from the furrows, which do not extend beyond the edge of the test 

 on the lower side, meeting the scattered pores which diverge from the ex- 

 tremity of the petals and extend to the edge of the test. The ambulacral 

 farrows are deep and broad as far as the first bifurcation. The spines are 

 short on the abactinal surface, somewhat club-shaped, and considerably longer, 

 more slender and cylindrical on the actinal side, corresponding to the smaller 

 or larger tubercles of the edge of the test and ambulacral furrows. According 

 to Stunpson and Martens the color in life is dark violet. 



The jaws are flatter and more slender than those of E. parma ; the con- 

 necting pillars and walls between the two floors extend nearly one half the 



