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PLATYCRINID.ZA. 649 
Type of the genus: Platycrinus levis Miller. 
fiemarks. — We have included under Platycrinus species with an anal 
tube, as well as those in which the anus opens directly through the disk. 
A tube, such as was represented by the Austins, and by de Koninck and 
Le Hon, extending almost to the tips of the arms, has only been observed 
among American species in P. excavatus. Most of them have an excentric, 
mammiform protuberance with a small opening in the middle. The longest 
tube except the above which we have found is that of P. burlingtonensis ; it 
rises but a few millimetres above the general level of the disk, and has 
a small opening at the end, while the end of the tube in some of the 
Kuropean species is closed, rounded off, and, according to Austin, valvate. 
The Austins made an unsuccessful attempt to subdivide the genus upon 
the structure of the anus. Under Platycrinus, they proposed to place the 
species with “a central elongated oral tube.” For species with a “central 
valvate, unobtrusive mouth, or mouth capable of being withdrawn into the 
a 
visceral cup,” they proposed the name “ Centrocrinus,” and “ Pleuroerinus” for 
those in which the mouth is “placed laterally, or not central.” Some of 
these ideas are altogether fanciful. The so-called “mouth,” for which these 
authors mistook the anus, is always excentric ; and a withdrawal of the anus 
into the body, as they imagined, is incompatible with the construction of the 
ventral side of Camerate Crinoids as now understood. Neither of the pro- 
posed names has been applied to any of the species, not even by the 
Austins; and it seems to us that, for the present at least, any general 
division based upon the structure of the disk and anus, is not practicable 
in a group in which the parts in question are but rarely observed. 
In default of any other characters upon which a generic division could be 
established, we have arranged the species into groups, in accordance with 
certain specific peculiarities, hoping thereby to assist the student in the 
identification of the species. 
Of the one hundred and twenty species of Platycrinus described from 
America, we recognize but sixty. Many are unquestionably synonyms. 
Others were made from such imperfect material, or so insufficiently de- 
scribed, that their identification is absolutely impossible. The outcome may 
be somewhat unsatisfactory, but it is the result of careful study and impartial 
consideration, and if we have erred in this respect it is probably in not 
carrying the process of elimination far enough. It may not be out of place 
to state that we devoted a long time to the revision of Platycrinus, and 
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