574 NORTH AMERICAN INDEX FOSSILS. 



in simple series, when one pair is placed over the other in suc- 

 cession from mouth to apex. They are biserial when placed so 

 that there are two vertical rows of pairs. Simple series of pores 

 are either absolutely straight or in curves or arcs of three or more 

 pairs. Ambulacra are simple when band-shaped and continuous 

 from center of top to center of bottom. Petaloid ambulacra are 

 those in which the pore-bearing zones of an ambulacrum separate 

 between the apex and the circumference {ambitus) of test and 

 contract again (petal-like) more or less perfectly before reaching 

 that region. Subpetaloid ambulacra are comparatively larger than 

 the petaloid, and the pairs of pores do not tend to close again 

 towards the ambitus. The pores almost cease at the end of the 

 petaloid parts, though a few can usually be traced farther, at times 

 to the mouth. The anterior ambulacrum of the irregular echinoids 

 is generally much less fully developed, and often lies in an anterior 

 sulcus {¥\g.igii,A,C,as). 



In some echinoids {e. g., Cassidulus) the group of ambulacral 

 plates bordering the mouth opening are petal-like and swollen 

 (phyllodes), with the pores crowded and prominent (Fig. 1928,^). 

 This forms the floscelle. The five phyllodes are separated by 

 inflated interambulacral plates called the bourrelets (Fig. 1930,0). 



The apical system (Figs. 1917, &; 1935,0) is usually composed 

 of ten plates arranged in two alternating circles of five plates each, 

 and each plate perforated. The uppermost circle is situated inter- 

 radially, and consists of large, five- or six-sided pieces, the basal 

 or genital plates (Fig. 1935 a, g) ; the largest of these, the madre- 

 porite, is a sieve-like prominence (Fig. 1935 a, m). The plate 

 lying to the front and on the left side of the madreporite is the 

 anterior ocular which lies upon the upper end of the odd or 

 anterior ambulacrum (Fig. 1935, &). The lower circle of smaller 

 plates upon the ends of the ambulacrum are the radial or ocular 

 plates (Fig. 1935 a, r). 



The plates are usually covered with tubercles and granules which 

 carry spines. The larger tubercles are called primaries. The base 

 or boss of the tubercle supports a rounded knob (mamelon) , which 

 is said to be perforated when pierced by a central foramen for a 

 slight distance, or imperforate when it is not so pierced. A plain 

 sunken space surrounding the base of the tubercle is called the 



