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HEXACRINID.Z. 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. 
Type of the genus: Pterotocrinus capitalis (Lyon). 
ftemarks. — This genus was originally described by Lyon under Astero- 
crimus, a name preoccupied by Miinster. 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 Tularocrinus, 
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 entirely, as accessory pieces. These plates, 
although present in every specimen, are in some cases completely covered 
by the distichals. 
Cyathocrinus protuberans 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 Platyceras, 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 hemisphericus and Gilbertsocrinus tuberosus, 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 hemisphericus, for the arms 
in some species of Péerotociinus are so short that they do not reach the 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 
Hexacrinide, constitute a part of the calyx proper. The genus has its closest 
affinities with Zalarocrinus, 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. Péerotocrinus, so far as we know, is the 
* To this fact Mr. Charles R. Keyes directed attention in his interesting paper, On the Attachment 
of Platyceras to Paleocrinoids (Proceed. Amer. Philos. Soc., Vol. XXV., p. 237). 
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