HEXACEINID^. 793 



Distribution. — Restricted, so far as known, to the Kaskaskia group of 

 North America. Detached appendages are found in large numbers in certain 

 localities of Alabama, Kentucky, Tennessee, and Illinois, but perfect speci- 

 mens are extremely rare. 



Tt/j)e of the genus: Pterotocrinus ccqntalis (Lyon). 



Remarhs. — This genus was originally described by Lyon under Astero- 

 crinus, a name preoccupied by Munster. Meek and Worthen, in 1866, in 

 revising the genus, stated that in some species the interradials rest upon the 

 superior lateral faces of the radials, which is the case in the allied Talarocrinus, 

 but not in Pterotocrinus. Wetherby regarded the small trigonal costals, 

 which Meek and Worthen identified as "second radials," and which Lyon 

 and Casseday had overlooked entirel3% as accessory pieces. These plates, 

 although present in every specimen, are in some cases completely covered 

 by the distichals. 



Cyailwcrinus protiiberans Hall very probably belongs to this genus, but as 

 only the basals and portions of radials are known, we are unable to describe 

 it satisfactorily. 



It is very interesting that the anus in almost every specimen of this 

 genus is covered with a Plati/ceras, and in every case the anterior margin 

 of the shell is directed to the posterior side of the crinoid, contrary to the 

 cases of Platycrinus kemisphericus and Gfilhertsocrinus tuherosus, in which the 

 anterior margin of the shell lies to the anterior side of the crinoid.* That 

 the Gasteropod invariably occupies the same position proves, we think, that 

 its presence there is the result of habit and not of accident. In Pterotocrinus 

 it could not have been washed in by the currents of the arms, as suggested 

 by Meek and Worthen in the case of Platycrinus Jiemispkericus, for the arms 

 in some species of Pterotocrinus a.\'Q so short that they do not reach tlie sum- 

 mit of the calyx. 



Pterotocrinus is an aberrant and highly differentiated form. It approaches 

 the typical form of the Camerata in the comparatively large size of the fixed 

 brachials, which to the third order, contrary to what is the case in all typical 

 Hexacrinidse, constitute a part of the calj'x proper. The genus has its closest 

 affinities with Talarocrinus, which precedes it in time, and is doubtless its 

 ancestral type. Their structural peculiarities tend in the same direction; 

 but while feebly indicated in the latter form, they attain in Pterotocrinus the 

 climax of extravagant development. Pterotocrinus, so far as we know, is the 



* To this fact Mr. Charles R. Keyes directed attention in liis interesting paper, On the Attachment 

 of Platyceras to Palteocriuoids (Proceed. Amer. Philos. Soc, Vol. XXV., p. 237). 



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