Vol. XXI, pp. 149-152 June 9, 1908 



PROCEEDINGS 



OF THE 



BIOLOGICAL SOCIETY OF WASHINGTON 



TWO NEW CRINOID GENERA. 



BY AUSTIN HOBART CLARK, 

 U. S. Bureau of Fisheries. 



A critical study of the arm structure and the brachial homol- 

 ogies in the recent representatives of the family Pentacrinitidae 

 has made it evident that we are at present confusing, under the 

 name of Isocrinus (= Pentacrinus of P. H. Carpenter) three 

 quite distinct types, one of which occupies, in certain ways, an 

 intermediate position between the others and Metacrinus. In 

 Metacrinus we have a condition similar to that found in the Pen- 

 tame trocrinidae, costals being absent altogether (see these Pro- 

 ceedings, antea, p. 134), the only difference being that in Meta- 

 crinus the arms branch, sometimes as many as five or six times, 

 while in the Pentametrocrinida? the arms are undivided. In 

 Isocrinus asteriu, I. blalcei, and I. decorus, costals are present, but 

 the arm distal to the first axillary is strictly comparable to the 

 entire arm in Metacrinus/ that is, in these three species, no matter 

 how many times the arms divide, they are strictly homologous 

 with the arms in a ten-armed comatulid (excepting, of course, 

 Deeametrocrinus and U intacr inns') , and are not at all homologous 

 morphologically (though strictly analogous physiologically) to 

 the arms of a multibrachiate comatulid. In Isocrinus alterni- 

 cirrus, I. pairse, I. sibogse and /. ivyinlle-thomsoni the arms are 

 morphologically and physiologically homologous with the arms 

 of multibrachiate comatulids, being composed of costals, dis- 

 tichals, pal mars, and one or two post-palmar series before the 

 undivided free arms are reached. 



The fact that the arm division in each case results in an isoto- 

 mous dichotomy appears to have led students to consider it as 

 strictly similar; but while in Isocrinus alternicirrus, I. pame, I. 



J2— Proc. Biol. Soc. Wash., Vol. XXI, 1908. (149) 



