SCHIZASTEB FRAGILIS. 75 



test from above is -till elliptical, with the exception of the anterior re-entering 

 angle of the odd ambulacrum and of the posterior extremity of the t< 

 The apex is now posterior tn the abactinal system, the anterior extremity 

 sloping gradually toward the ambitus, while the posterior extremity of the 

 tesl extends nearly horizontally to the junction of the vertically truncated 

 posterior extremity. 



In specimens measuring i!-"- nun. in length the outline of the tesl when 

 seen from above shows traces of the angular projections which characterize 

 the full-grown specimens. The tesl 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 shori distance 

 from the ambitus to the apex. The lateral ambulacra are now all sunken, 

 the posterior ambulacra somewhal less than the anterior pair; these have 

 now eleven pairs of pores. There are three large genital openings, the two 

 lefl anterior ones and the righi 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 he round in specimens measuring about 10 mm. 

 in length. The tuberculatum and the corresponding spines have now as- 

 sumed the general characteristics of the tuberculation at dififerenl parts 

 of the test; while in younger stages the tubercles were few in number, 

 comparatively large, and irregularly arranged over the -ides and upper 



.indcc of the test. The actinal plastron is the firsl to appeal', and ill the 



voungesl specimens examined the tubercidation of that area as well as 

 the spines had already become specialized to a certain extent. 



In speci ns measuring •"> I 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 ami shape of 

 the fascioles, the outline of the tesl as -ecu front different points of view, 

 and the specialization of the spines of different parts of the test, a- we do 

 in the larger specimens. 



