SCHIZASTEE FRAGILIS. 75 
test from above is still elliptical, with the exception of the anterior re-entering 
angle of the odd ambulacrum and of the posterior extremity of the test. 
The apex is now posterior to the abactinal system, the anterior extremity 
sloping gradually toward the ambitus, while the posterior extremity of the 
test extends nearly horizontally to the junction of the vertically truncated 
posterior extremity. 
In specimens measuring 23 mm. in length the outline of the test when 
seen from above shows traces of the angular projections which characterize 
the full-grown specimens. The test is slightly pointed posteriorly. The 
odd ambulacrum is deeply sunken nearly to the apical system, while in the 
preceding stages the odd ambulacral groove extended only a short distance 
from the ambitus to the apex. The lateral ambulacra are now all suid\:en, 
the posterior ambulacra somewhat less than the anterior pair ; these have 
now eleven pairs of pores. There are three large genital openings, the two 
left anterior ones and the right posterior one. The fascioles have nearly 
assumed the course and shape they take in the adult. The posterior ex- 
tremity is bevelled towards the actinal extremity, so that over the anal 
system the test projects slightly beyond the general outline. The actinal 
system is now quite flat, the actinostome has a well-developed posterior lip, 
of which a trace only can be found in specimens measuring about 10 mm. 
in length. The tuberculation and the corresponding spines have now as- 
sumed the general characteristics of the tuberculation at different parts 
of the test; while in younger stages the tubercles were few in number, 
comparatively large, and irregularly arranged over the sides and upper 
surface of the test. The actinal plastron is the first to appear, and in the 
youngest specimens examined the tuberculation of that area as well as 
the spines had already become specialized to a certain extent. 
In specimens measuring 34 mm. in length, the test, as seen from above 
and in profile, has assumed the outline of adult specimens ; and we find 
already in specimens of this size as much variation in the closeness of the 
tuberculation, the depth of ambulacral furrows, the course and shape of 
the fascioles, the outline of the test as seen from different points of view, 
and the specialization of the spines of different parts of the test, as we do 
in the larger specimens. 
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