NATURAL SCIENCES OF PHILADELPHIA 161 



into a double series of small interlocking pieces supporting numerous tenta- 

 cles. Other species, however, generally included in the genus, have no pro- 

 boscis, 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 especial- 

 ly coincident with apparently any one of the other characters mentioned. 



As defined by Koninck and Le Honf , in accordance with their improved 

 nomenclature of the parts, the structural formula of this group is as follows : 



Basal pieces, 3 ; forming a wide cup. 



Radials, 2 ; one large aud one small. X 5. J 



Anals, 1 large, or 3 small. 



Interradials, 1, X 4. 



Arms, 10, 20, 25, 30 or 35, according to the species. 



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

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

 the Messrs. Austin, $ into the following four sections : 



1. Platgcrinus, (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-dactylug, Austin ; 

 P. Miillerianus, Koninck; and P. granulatus, Miller. 



2. Centrocrinus, Austin. Opening of summit nearly or quite central, but 

 not elevated upon a proboscis. 



Type. P. \_Centroer.~\ gigas, Gilbertson. 



3. Cupellseocrinus, Troost. Differs from the last 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. P. Tennesseensis , Roemer. 



4. Pleurocrinus, Austin. Diifers from Centrocrinus only in having the open- 

 ing of the summit lateral, and nearly or quits on a line with the arm bases 



Examples. P. [Pleurocr.~\ mucronatus, Austin;' P. \_Pleurocr .] tuberculatus, 

 Miller; P. [Pleurocr.~\ tub eros us, and P. [Pleurocr.~\ subspi?iosus,H>i\l; P. [Plev- 

 rocrA asper, Meek & Worthen, &c, &c. 



In regard to the value and importance of the characters distinguishing these 

 sections, Palaeontologists will probably always differ. Hitherto these differ- 

 ences have scarcely been noticed, even by the most respectable authorities, 

 excepting as one of the means of distinguishing species. From all analogy, 

 however, it seems reasonable to suppose that they were accompanied by cor- 

 responding modifications in the structure of the softer parts of the animal. It 

 will also be observed, that they correspond, in part, almost exactly to the charac- 

 ters distinguishing sections of the allied Aclinocrinus group. For instance the 

 species embraced in the section Pleurocrinus, differ from the typical forms of 

 Platgcrinus, almost precisely as Agaricocrinus and Amphoracrinus do from the 



*P. nodobrachialus, Hall, is an American example with the arms composed of a single series of 

 pieces. We allude here to a species described under that name by Prof. Hall, in the Iowa Report, 

 p. 542, 1S58, aud not to another form described by him under the same name, in his " Descriptions 

 of New Species of Crinoidea, Ac." Albany, Feb. 25, 1851, p. 17. The inconvenience and confusion 

 liable to result from the use of the same specific name for two forms of the same genus, makes 

 it necessary that another name should be applied to one of these species ; hence we would propose 

 to call that described at the latter date, P. perasper. 



fRecherches Sur les Criuoides du Terrain Carbonifere de Le Belgique, p. 155, 1854. 



Jit is worthy tf note that although Koninck and Le Hon give two radials (one large and one 

 small) as the number, that their figure 1 a, pi. vi. of P. Ixvis, Miller, shows clearly 3 radials, 

 one large and two small. As others figure and describe it as having only two. this may be only 

 an accidental variety. It will be seen, however, that our P. parvulus, described on another page 

 of this paper, has 3 radials, one large and two small. Still two seems to be the normal number 

 in this genus. 



J Monograph of Recent and Fossil Crinoidea, p. 6. 



1865.] 



