METALIA STERNALIS. 601 



gate, slightly diverging, running parallel to the narrow interambulacral space 

 which separates them. At about one third their length, the inner poriferous 

 zone, which remains thus far rudimentary, reduced to a mere line of distant 

 pores, increases rapidly in width, the whole course of the petal is deflected 

 outwardly, and at about half the length of the petal the two poriferous zones 

 are of equal width, and, remaining so, run almost parallel to the extremity 

 where the poriferous zones become somewhat narrower at the rounded end 

 of the petal. The outer portion of the petal curves slightly inward. The 

 aetinal surface of the large specimens is convex, flattened in the median 

 interambulacral space, and rounded towards the edges, especially in the lat- 

 eral posterior interambulacral spaces. The flat aetinal plastron is narrow, 

 elongate, rounded at the two extremities, with a slight keel, forming two 

 low nodes at the posterior edge of the two principal plates composing it. 

 It is edged by the narrow, bare, posterior lateral ambulacra, which extend 

 till they meet the subanal plastron, then diverge, following the line of the 

 plastron, and are lost in the tubercles before reaching the edge of the test. 



The subanal plastron is broad, heart-shaped, occupying nearly the whole 

 of the posterior part of the sloping aetinal surface of the test. It is edged 

 by a very broad fasciole, broadest at the posterior edge, sending off a narrow 

 anal branch, which remains open, but extends to the abactinal surface above 

 the anal system. The anal system, situated in the upper part of the de- 

 pression of the posterior part of the test, is elliptical longitudinally, pointed 

 above and below ; the posterior part of the subanal fasciole is on the edge 

 of the depression of the test in which the anal system is placed. 



The peripetalons fasciole forms across the posterior interambulacrum and 

 the posterior lateral interambulacral spaces an open rectangle, with rounded 

 corners and slightly undulating sides. The width of the fasciole equals in 

 breadth the median interporiferous space of the posterior petals. It forms 

 a right angle parallel to the anterior petals, at a short distance from it, and 

 then runs diagonally towards the extremity, which it crosses at right angles 

 to the trend of the petal, then diverges at the same angle on the other side 

 of the petal, forming a deep re-entering angle in the anterior lateral inter- 

 ambulacra. It then runs obliquely towards the ambulacral furrow, and at 

 a distance from it runs at a less angle towards the median line of the odd 

 ambulacrum. The actinostome is transverse ; the phyllodes well developed. 



With the exception of the short diagonal rows of somewhat large tubercles 

 along the edges of the petals and of the odd ambulacrum, within the peri- 



