INFRABASALS IN EECENT GENEEA OF THE CRINOID 

 FAMILY PENTACRINITID^E. 



By Austin Hobart Clark, 



Assistant, Bureau of Fislicrics. 



Althoiig-h so long ago as 1885 Wachsmiitli and Springer showed 

 chat Isocrimis and Metacrinus are constructed upon the dicj^clic prin- 

 ciple, infrabasals have as yet never been detected in any species of 

 Metacri7ius nor in any recent species of Isocrinus. It is now known 

 that all recent crinoids, with the single exception of Hyocrimis, are 

 dicyclic; but infrabasals have never been actually demonstrated 

 except in two species, in Antedon hifida by Bury, and in Calamocri- 

 nus diomedce by Alexander Agassiz. 



Dr. P. H. Carpenter in his monograph on the " Comatulae " criti- 

 cizes rather sharply the so-called law of Wachsmuth and KSpringer 

 for determining by the orientation of the stem whether the infra- 

 basals are present or not in a given species, and positively asserts that 

 they do not exist in the recent Pentacrinitida^, although he admits 

 that they occur in the liassic genus Pentacrinus (i. e., " Extmcrinus "). 



The discovery of infrabasals in Isocrinus was made by the distin- 

 guished paheontologist P. de Loriol who, in 180-i, described a new 

 species of the genus, Isocrinus leuthardi, and published figures of it 

 show^ing the five small radially situated infrabasals occupying a posi- 

 tion in the center of the star-shaped figure formed by the elongate 

 basals. 



With the idea of determining whether the condition shown by 

 de Loriol in Isocrinus leuthardi was repeated in the recent members 

 of the genus and in Metaciinus, preparations were made of Isocrinus 

 decorus and M etac7'i7ius rotvndus by carefully removing the upper 

 stem joints so as to lay bare the entire dorsal surface of the basals, 

 and it w^as with considerable surprise that in both species prominent 

 infrabasals were revealed, those of Metacrinus. rotundus especially 

 being so noticeable that it is considerable of a mystery how they could 

 possibly have escaped the notice of such a careful worker as Doctor 

 Carpenter. 



Proceedings U. S National Museum, Vol. XXXIII— No. 1582. 

 Proc. N. M. vol. xxxiii— 07 43 671 



