14:2 Proceedings of the Biological Society of Washington. 



family Mariametridse, the first three arc homogeneous and 

 satisfactory units, hut the last is quite heterogeneous and far 

 from satisfactory, having been used, in fact, to include all such 

 forms as are referable to the Mariametridse but which do not 

 fall within the scope of the others. 



It is possible by the division of the genus Dirhrometra as now- 

 understood into three natural sections to so rearrange the various 

 species that they will fall into groups which will he found to be 

 strictly comparable to the other genera in the family Mariame- 

 tridse, and to the genera in the most nearly related families. 



The lines along which the generic divisions within the family 

 Mariametridse would then fall are indicated by the following 

 key: 



KEY TO THE GENERA OF MARIAMETRIDSE. 

 (i) Cirri very large and long, with more than 40 (usually 50-80) seg- 

 ments 

 A' 1'] on the ( inter arms greatly elongated and flagellate, more than 

 five times as long as the following pinnules; 

 division series very narrow, very deep dorso- 

 ventrally, and very widely separated 



( 1 ) Pontiometra 

 lr l'i only very slightly, if at all, longer than 1*-, and of the same 

 character; division series nearly or quite in 

 apposition laterally through the extension of 



their ventrolateral borders 



(2) Oxymetra 



a 2 cirri short or of moderate length, with less than 40 segments 



/•/ Pa and Ps similar, and of equal length 



C!) Liparometra (nov.) 



'/-' P« and P.; of unequal length 



c 1 ]'» markedly longer, and more or less stouter, than P;or Pi 



( 4) Lamprometra (nov. ) 



>■- P : ; markedly longer than Pl<, which in turn is markedly 



longer than Pi 



d ] lateral portions of the dorsal surface of the division 



series perfectly smooth; division series never 



carinate, and never with a narrow dark median 



line; disk naked 



(5) Dichronn tra 



tP lateral portions of the dorsal surface of the division 

 series with a prominent granular, tubercular or 

 spinous ornamentation; a faint median carina- 

 tion or a narrow dark median line on the division 

 series and arm bases, often both occurring to- 

 gether: disk completely covered with calcareous 



platen 



(6) Mariametra 



