PLATYCEINID^. 649 



Type of the genus : Platycrinus Icevis Miller. 



Bcmarlcs. — 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. excavahis. 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. hurlingtonensis ; 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 

 European 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 valvate, unobtrusive mouth, or mouth capable of being withdrawn into the 

 visceral cup," they proposed the name" Centrocrimis" and " Pleurocrinus" 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 defiiult 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 Plati/crinus 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 



