196 PEOr. p. MAT?TIN DUNCAN ON THE 



3, aud 4 in the zones. But the correct numbering must follow on 

 the examination of some of the other groups, for, as suggested 

 in Salmacis, the single plate was really not such a structure 

 originally.) 



A large form, closely allied to the species just considered, has 

 tall and broad processes, which are expanded laterally on each 

 side of the vertical median suture above. The opening they en- 

 close is a tall triangle in shape with the angles rounded. The 

 ridge is very low, much curved downwards at the free edge, and 

 marked on each side of the bulge of the base of the tubercle- 

 bearing plates at the peristome by the groove leading to the 

 branchial cuts. The bulge is not simple however, and it is made 

 up of two sides with a median depression. The sutures between 

 the processes and the ridges are very distinct. 



Young Form. — The growth of the perignathic processes was 

 attempted to be understood by the examination of a number of 

 small specimens of IE. norvegicus, varying from 5 to 8 mm. in dia- 

 meter. In the smallest form the plates of the test were few, and 

 those of the ambulacra were distinct and wide apart. The pro- 

 cesses were the merest nodules, were widely separated, short, and 

 with a narrow and almost circular base (fig. 21) ; they were 

 united by suture to the ridges, and the line of union was distinct 

 even without benzene, so that it was perfectly evident that the 

 stunted growths were not on interradia but on ambulacra (fig. 23). 

 Every process was a portion of the first ambulacral plate, ou 

 either side of the median line at the peristome, and it was 

 evident that the position was on the outer poriferous portion of 

 the plate for the first pair. The first pair of pores were pushed 

 towards the median line by the base of the process^ and the aboral 

 pore of the pair perforated the base of the process (fig. 17), 



The position of the base of the process was then on the pori- 

 ferous zone close to the edge of the peristome, and close to the 

 arubulacro-interradial suture. It is proved, therefore, that the pro- 

 cess is not homologous with the so-called " vertebral processes " of 

 Miiller, which are growths of interporiferous areas, and there can 

 no longer be a doubt that the processes are ambulacral growths aud 

 not interradial. Even at this early age the processes had feeble 

 muscular slips. The ridges were very low and insignificant, and 

 their edge was composed of an entire plate as in adults, and it 

 was a miniature of the ridge already described and drawn in 

 fig. 24. Slightly larger specimens showed the processes to be taller 



