318 
K.J. McNamara 
Description 
Test tumid (Figure 1A,C), reaching up to 33.5 
mm TL; anterior notch moderately developed 
(Figure 1A,B); highest posteriorly in 
interambulacrum 5, just posterior of apical system, 
forming a prominent keel (Figure 1C); height 63- 
72%TL (mean 67.5;SD=2.6; n=23); wider than long, 
ranging between 87-97%TL (mean 91.7; SD=4.6; 
n=26). Position of apical system variable; 
posteriorly eccentric, 56-65%TL (mean 60.0; 
SD=2.4; n=25) from anterior ambitus; ethmolytic, 
with two to four genital pores (Figure 3) (see 
below). 
Ambulacrum III relatively broad (Figure 4A), 18- 
25%TL (mean 20.8; SD=1.7; n=25); moderately 
deeply incised (Figure 1A), shallowing slightly near 
ambitus; bears up to 38 pore pairs; smallest with 18 
(Figure 5C); within a pair each pore separated by 
raised interporal partition; pores almost 
transversely aligned in a pair, although becoming 
slightly oblique abapically. Anterior petals 
moderately deep; broad, width 9-13%TL (mean 
11.1; SD=0.87; n=26); diverge anteriorly at about 
80° (Figure 1); slightly flexed distally; relatively 
long, 33-42%TL (mean 36.3; SD=2.0; n=26); bear up 
to 31 pore pairs (Figure 5B); pores within each pair 
widely spaced, elongate, not conjugate. Posterior 
petals slightly shallower and much shorter than 
anterior pair (Figure 1A), length 12-21%TL (mean 
17.5; SD=1.8; n=25); width 8-10%TL (mean 8.7; 
SD=0.67; n=25); bear up to 20 pore pairs in each 
row (Figure 5A). Peripetalous fasciole very broad 
(Figure 1A) and moderately indented between 
anterior and posterior petals. Lateroanal fasciole 
very thin. 
Peristome lunate (Figure IB), width 14-20%TL 
(mean 17.4; SD=1.6; n=24); slightly sunken; situated 
16-24%TL (mean 19.0; SD=2.1; n=24) from anterior 
ambitus. Labrum narrow and long; anteriorly 
acuminate medially and projecting antero-ventrally 
(see McNamara and Philip 1980a, Figure ID). 
Phyllode with unipores: 10 in ambulacra II and IV; 
5 in ambulacrum III; 6 in ambualcra I and V. 
Periporal area slightly swollen around abambital 
unipores, but flat around adambital unipores. 
Plastron slightly convex; width 38-46%TL (mean 
41.9; SD=2.1; n=23). Periproct ovate; length (Figure 
6D) 13-21 %TL (mean 16.3; SD=1.6; n=26); width 
(Figure 6C) 11-15%TL (mean 11.4; SD=0.92; n=26). 
Ontogenetic Variation 
The 26 specimens of Schizaster compactus range in 
test length from 12.1 mm to 33.5 mm (Figure 7). 
Mean length is 24.8 mm (SD=4.6). The smallest 
specimen has unopened gonopores and therefore 
can be considered to be a juvenile. Gonopores open 
between 12 mm and 17.7 mm, the next largest 
specimen being 17.7 mm in test length, having fully 
opened gonopores. 
Of the characters measured on the aboral surface 
of the test, significant ontogenetic changes occur in 
the width of ambulacrum III, the length of the 
A 
Figure 2 Arrangement of pore pairs in ambulacrum III aborally in the three subgenera of Schizaster: A, S. (Paraster); 
B, S. (Schizaster); C, S. (Ova). 
