188 SYNOPTIC COLLECTION. 



Toxaster oblongus Deluc. (No. 350), is longer than 

 broad with a groove at the anterior end, the posterior end 

 being high with the anus visible. 



Holaster striato-radiatus d'Orb. (No. 351), is a high 

 dome-shaped Spatangoid with simple narrow ambulacra 

 and broad interambulacra. The mouth is at the anterior 

 end of the ventral side and the anus at the posterior 

 end. The abactinal area cannot be made out in the 

 specimen but the boundary lines of some of the plates 

 can be traced. 



Colly rites dorsalis Ag. (No. 352), shows more plainly 

 the specialization in the position of the ambulacra, three 

 of which are in front while the other two are widely sep- 

 arated from them. This drawing out of the abactinal 

 area in an antero-posterior direction has caused a sep- 

 aration in the genital and ocular plates. 



No. 353 is an interesting specimen of Micraster from 

 the Lower Greensand. It is a jasper cast in which the 

 posterior end is preserved. The partition between the 

 paired pores is clearly shown and three of the ambulacra, 

 while the ornamentation is well preserved. This genus 

 is usually distinctly heart-shaped with the bilabiate mouth 

 placed far forward and the odd anterior ambulacrum in a 

 groove. 



Hemiaster minimus Desor (No. 354), when young has 

 the anus nearly central and the test has much the 

 appearance of that of the regular urchins. Marked 

 changes take place, however, in the course of develop- 

 ment. The outline of the urchin becomes more irregular 

 and flattened, and some of the ambulacral plates become 

 modified into four deep cups or pouches for the purpose 

 of holding and protecting the eggs. Within these pouches 

 the embryos develop; by the law of acceleration the meta- 

 morphosis is skipped, and the embryos are retained by 

 the parent until the plates of the test are formed. The 

 adult has a band of microscopic tubercles called fascicles 

 encircling the petals. 



