328 ART. 2. — H. MATïUMOTO : 



pericalles Clark, the peristomial plates are double, without the 

 unpah'ed secondary plate. The oral frames are not very stout 

 and ^have more or less distinct lateral wings, which are less well 

 developed than in Ophionereis and Ophiocrasis. The dorsal surface 

 of the vertebrœ is notched at the inner end so as to be fairly Y- 

 shaped, but not so strongly as in Ophionereis and Ophiocrasis. 

 The radial shields, genital plates and scales are similar to those 

 of Ophionereis and Ophiocrasis. 



As to the biological significance of the characters of the 

 present family, we can recognise two lines along which advance 

 has been made from the Ophiochitonime to the Ophionereidinœ. In 

 the first place, the Opl doner eidince have better developed chewing 

 apparatus than the Ophiochitonince, such as stouter teeth, with 

 widened and truncated end, smaller peristomial plates, stouter 

 oral plates with well developed lateral wings and more voluminous 

 muscles. In the second place, the arms of the Opliionereidince are 

 more flexible than those of the Ophiochitonince, as may be judged 

 from the notched, V- or Y-shaped dorsal surface of the vertebrae. 

 It may be remarked that the arms of Ophionereis and Ophiocrasis 

 are strongly flexed in alcohol and very freely mobile in life. 



Key to genera of Ophiochitoninœ, 



A — Disk entirely free of grannies ; arms markedly keeled above and 



below Ophiochiton. 



A A — Disk or at least the interbrachial ventral surfaces covered with fine 

 granules ; arms not keeled, at least below Ophioplax. 



Ophiochiton fastigatus Lyman. 

 OpMocMton fastigatus : Lyman, Bull. Mus. Comp. Zool., V, 1878, p. 



