Geology. 405 



of the present subclass from the asteroids depends merely upon 

 the different development of certain common structures. 



"Subclass IL Myophiuroida Matsumoto. Ophiuroidea with- 

 out external ambulacral grooves, and with ventral arm plates. 

 Kadial 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 

 vertebrae. Madrejjorite 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," (pp. 5-6.) 



"Though it is my purpose to discuss the results of a study of 

 Palaeozoic ophiurans in a future paper, I will here enumerate 

 some of the more important structures of Palaeozoic Myophiu- 

 roida, as bearing on the question before us. 



"1. Disk covered with delicate scales or by a naked skin, with- 

 out distinct primaries. 



" 2. Radial shields absent. 



" 3. Genital plates and scales absent. 



"4. Oral shields absent. 



" 5. Adoral shields not very distinctly specialized from the 

 lateral arm plates. 



" 6. Oral plates and frames long and slender. 



" 7. Distinct creases probably present between the interbrachial 

 ventral surfaces and arm bases. 



" 8. Dorsal arm plates entirely absent, or present only in a few 

 basal joints ; the dorsal side of the arms therefore largely unpro- 

 tected. 



" 9. Lateral arm plates with prominent spine ridges, which 

 extend to the ventral side of the arm ; those of the two sides not 

 meeting above or below, except in the very distal arm joints. 



" 10. Ventral arm plates higher in position than the lower bor- 

 ders of the lateral arm plates_, so that the arm is longitudinally 

 grooved ventrally. 



" I believe that the Palaeozoic Myophiuroida are the stock from 

 which the recent ophiurans have been directly derived, because 

 they show no trace of peculiar specialization and are fairly inter- 

 mediate in their organisation as a whole between the CEgophiu- 

 roida and recent ophiurans. If this view be right, then the most 

 archetypal group of recent ophiurans must be looked for among 

 those forms which have the strongest resemblances to the Palaeo- 

 zoic Myophiuroida." (pp. 367-369.) c. s. 



2. Puhlications of the United States Geological Survey; 

 George Otis Smith, Director. — Recent publications of the Sur- 

 vey are noted below. See earlier May, 1917, pp. 418, 419 ; June, 

 1917, pp. 489, 490. 



