6 ART. 2. — H. MATSUMOTO : 



the present subclass from the asteroids depends merely upon the 

 different development of certain common structures. 



Subclass II. MYOPHIUROIDA Matsumoto. 



Opliiuroidea without external ambulacral grooves, and with 

 ventral arm plates. Eadial shields, genital plates and scales, oral 

 shields, peristomial plates and dorsal arm plates usually present ; 

 but sometimes, some or all of them may be rudimentary or 

 absent. Ambulacral plates opposite, usually completely soldered in 

 pairs to form the vertebra?. Madreporite represented by one, or 

 sometimes all, of the oral shields. 



This subclass includes certain Palaeozoic forms and all the 

 ophiurans from the Mesozoic downwards. 



The Palaeozoic Myophiuroida appear to me to represent a dis- 

 tinct order by themselves ; but I defer the question to a future 

 paper. 



Order i. phrynophiurida Matsumoto. 



Disk and arms covered by skin. Eadial shields either very 

 rudimentary or long and bar-like. The radial shield and genital 

 plate of either side of each radius articulate with each other by 

 means of a simple articular face or a transverse ridge of either 

 plate. Peristomial plates large, entire, double or triple. Oral 

 frames entire, without well developed lateral wings. Lateral arm 

 plates subventral or ventral in position, while the dorsal arm 

 plates are either entirely absent or rudimentary, so that the dorsal 

 side of the arms is mostly unprotected. Vertebral articulation 

 zygospondyline or streptospondyline. 



