282 BULLETIN 82, UNITED STATES NATIONAL MUSEUM. 



units in which the cirri are most uniform and most distinctive also have the most 

 uniform and distinctive adambulacral plates, and the reverse. In certain groups 

 a single specimen often has two or more types of cirri ; similarly, a single specimen 

 has two or more types of adambulacral plates; also, in the case of both structures, 

 most marked in the HeliometriniE, there may be great variation in different speci- 

 mens of the same species. 



In their relation to calcification the ambulacral lappets of the pinnules and 

 the adjacent lateral perisome show three chief conditions. 



In the 203 species examined the ambulacral lappets contained each a single 

 rod or spicule in 65 specimens, representing the following families : 



Comasteridse (very rare). Colobometridie (majority). 



Zygometridse (rare). Tropiometridae (rare). 



HimerometridEe (rare). Antedonidse (majority). 



Mariametridaa (rare). Pentametrocrinidae (alternative). 



In 60 specimens there were no definite ambulacra! structures whatever. These 

 were distributed in the following families: 



Comasteridse (great majority). Mariametridae (majority). 



Zygometridse (majority). Colobometridse (about half). 



Himerometridce (minority). Antedonidse {Antedon onlj'). 



Stephanometridse (all). Pentametrocrinidae (one species). 



In 52 species very highly developed side and covering plates, always of the 

 same general type, were present. These species included all of those examined 

 in the families Calometridse, Thalassometridre, Charitometridap. 



In the remaining specimens very diverse conditions were found, which, how- 

 ever, were always traceable to one or other of the two last. 



In the families Calometridae, Thalassometridae. and Charitometridse the side 

 plates possess certain distinctive features by which the species may be assignee 

 to their proper family. 



In the Calometridae there is a remarkable uniformity of structure throughout 

 the genera and species, but in the other two families there is considerable variation. 



In the Thalassometridae the species of the subfamily Ptilometrinae and of the 

 genera Stylomefra and Oeeanometra appear to possess distinctive side and co\'er- 

 ing plates, while the same is true of the species of Monachometra and Poecilo- 

 vietra in the Charitometridse; but in the other genera no distinctive features are 

 evident, though certain peculiarities, such as the possession of supplementary side 

 plates, are confined to limited groups of genera. 



The side plates of Pontiometra appear to be distinctive; but only one speci- 

 men was examined, and it is quite possible that they may be subject to more or 

 less extensive variation. 



The Pentametrocrinidae have a curious combination of a rod and a plate which 

 is diagnostic, though in certain parts of each individual rods alone may be found. 



Sarametra possesses a quite unique type of adambulacral plating, which is evi- 

 dently a modification of the equally unique type found in Zenometra^ both of which 



