370 ART. 2. — H. MATSÜMOTO : 



widely separated from one another by the lateral arm plates, which 

 are distinctly in contact with each other in the ventral median 

 line. This condition is also observed in the distal arm joints of 

 the Ophiomyxince ; so that, following the law of localised stages, 

 it is evidently more distinctly embryonal than that in which the 

 lateral arm plates are separated by the ventral arm plates. The 

 Œgophiuroida had exposed ambulacral grooves ; and this groove 

 became covered over by the ventral arm plates for the first 

 time in the Palaßozoic Myophiuroida ; so that, the meeting of 

 the lateral arm plates in the ventral median line can not be a 

 primary condition. Moreover, the assumption that the meeting of 

 the lateral arm plates in the ventral median line is a primary 

 condition, makes it very difficult to assign a probable cause of the 

 acquisition of the ventral arm plates. I therefore think that, 

 the separated condition of the lateral arm plates in the ventral 

 median line is an archetypal but not embryonal feature, while the 

 meeting of those plates is an embryonal but not archetypal 

 feature. 



In the Ophiomyxince, the lateral arm plates of the two sides are 

 widely separated from each other on the dorsal side, while in the 

 majority of the Ophiacauthidce, they are well in contact. The 

 second condition is what occurs also in the very distal arm joints 

 of the Ophiomyxince, as well as of many Palaeozoic ophiurans ; so 

 that, following the law of localised stages, it is evidently more 

 distinctly embryonal than the first condition. But, the assumption 

 that the meeting of the lateral arm plates in the dorsal median line 

 is a primary condition makes it very difficult to assign the neces- 

 sity for the acquisition of the dorsal arm plates. I therefore think 

 that, the meeting of the lateral arm. plates in the dorsal median 

 line is an embryonal but not archetypal condition, while the 



