RHYNOBRISSUS PYRAMIDALIS. 591 



The lateral ambulacral petals are short, extending about two thirds 

 towards the edge from the apex ; they are limited by the peripetalous fas- 

 ciole, which follows in a general way the outline of the test, except the 

 slight indentation in the posterior lateral ambulacra. The lateral ambulacra 

 are somewhat sunken, petaloid, remarkable for the great width of the porif- 

 erous zones, which leave only a narrow interporiferous space, a mere thread, 

 carrying a few distant miliaries, not a quarter of the width of the adjoining 

 poriferous zone ; the petals are straight, rounded anteriorly ; the pores large, 

 elliptical, nearly of the same size ; the inner pores the larger, the posterior 

 petals the longest; anterior petals diverging slightly anteriorly ; the anterior 

 poriferous zone more arched than the posterior one. The odd anterior ambu- 

 lacrum is flush with the test, narrow, consisting of hexagonal or pentagonal 

 plates, with minute pores in the median angles of the junction of the plates. 

 The tubercles (imperforate, not crenulate) of the whole of the upper part 

 of the test above the ambitus are uniform, closely packed, with distinct 

 raised scrobicular circles, and few most minute miliaries irregularly scattered. 

 Near the apical system, in the upper part of the odd ambulacrum, we find a 

 cluster of rather larger tubercles, carrying spines considerably larger than 

 those of any other part of the abactinal surface ; a narrow line along the 

 horizontal and vertical sutures is left bare. Within the peripetalous fasciole 

 the tubercles increase in size immediately adjoining the poriferous zones ; 

 and the spines are large enough to cover the whole width of the petals. The 

 spines are slender, short, curved, of uniform size over the whole abactinal 

 surface, except within the fasciole, as mentioned above, and towards the lower 

 side of the posterior extremity outside of the anal fasciole, where the tuber- 

 cles increase in size, and carry larger and longer spines, but similarly curved 

 to the shorter ones. The tubercles gradually increase in size, and reach 

 their maximum adjoining the bare actinal ambulacral avenues. In the an- 

 terior part of the test the tubercles increase rapidly in size to the edge of 

 the bare areas round the actinostome. In the actinal plastron they are 

 largest on the edge near the anterior extremity, diminishing in size towards 

 the keel and the posterior extremity. The spines of the actinal surface are 

 very long ; those of the actinal plastron are arranged in diagonal rows, 

 curving towards the central part of the plastron ; those of the lateral pos- 

 terior ambulacra are slightly S- S h a p et b curving outward at the extremity. 

 The ambulacral avenues are covered by slender curved miliary spines, 

 carried by miliary tubercles somewhat irregularly and clo>elv scattered over 



