172 PALAEONTOLOGY OF ILLINOIS. 



allied species, the body is more or less hemispherical below the arms, while the 

 dome terminates above in a long, generally .stout, central or subcentral probos- 

 cis, closed at the summit, but apparently pierced by a small aperture on one 

 side, near the upper extremity. In these typical forms the arms bifurcate 

 once, or oftcner, near the body, beyond which they are simple, and composed 

 at first of a single series of wedge-shape pieces, passing more or less gradually 

 into a double series of small interlocking pieces, supporting numerous tentacles. 

 Other species, however, generally included in the genus, have no proboscis, but 

 a simple aperture in the summit, located either laterally or nearly centrally ; 

 while some of these have the arms composed of a double series of interlocking 

 pieces, and others of a single series of wedge-shaped pieces,* neither of these 

 peculiarities in the structure of the arms being always especially coincident 

 with apparently any one of the other characters mentioned. 



From the foregoing remarks it will be seen that the group, including species 

 agreeing with the given formula, may be divided, as (in part) suggested by 

 the Messrs. Austin, into the following four sections if 



1. PIjATYCKINTTS, (typical.) 



With the summit terminating in a more or less elongated, central or subcentral 

 proboscis, bearing the opening on one side, near the upper extremity. 



Type. P. Ixvis; Miller; also includes P. spinosus and P. 30-dac/yliis, Austin ; 

 P. Mullerianus, de Koninck, and P. granulatus, Miller. 



2. CENTROCRIIVUS, Austin. 



Opening of summit nearly or quite central, but not elevated upon a proboscis. 

 Type. P. [Centrocr.J gigas, Gilbertson. 



3. PLEVBOCRINVS, Austin. 



Differs from Centrocrinus mainly in having the opening of the summit lateral, 

 and nearly or quite on a line with the arm bases. 



Examples: P. \Pleurocr.~\ mucronatus, Austin; P. \_Pleurocr.~] tuberculatus, Mil- 

 ler ; P. \Pleurocr.~\ tuberosus, and P. \Pleurocr .] subspinosus, Hall ; P. (Pleu- 

 rocr.) asper, Meek and Worthen, etc., etc. 



4. inABSIIJPIOGRINITES, Phillits. 



a — Differs from Platycrinus proper only in having its second radial pieces merely 

 rudimentary, or so small as to allow the first brachials to rest partly upon 

 the first radials. 



Type. M. eselatus, Phillips. 



b. — {Cupellxocrinus, Troost,) Differs from the last only in having a subcentral. 

 opening without a proboscis. 



Type. P. Tenncsseensis, Roemer. 



* P. nodobrachiatus, Hall, Iowa Report, p. 542, 1858 (not P. nodobrachialus, Hall, 

 1861 ;=P. perasper, M. and W., 1865), is an American example of this genus, with a 

 single series of pieces in each arm. It is worthy of note, that the type of Edwardso- 

 crinus, d'Orbigny, has arms composed each of a single series of pieces, though it was 

 not upon this character, but upon the erroneous supposition that it had five basal 

 pieces, that he proposed to separate it from Platycrinus. 



j- Monogr. Recent and Fossil Crinoidea, p. 6. 



