OP ARTS AND SCIENCES. 17 



wliile in D. cornigerus and D. sexlohatus, now described for the first 

 time, the whole number of radials is only five." 



In a number of well-preserved individuals of D. polydactylus, we 

 have always found three radials, and on examination of other species 

 we have concluded this to be the number most frequently met with. 

 The following list contains all the species of Dichocrinus which show 

 in a satisfactory manner all the radial pieces. 



D. cornigerus . . . . 1X5 



D. ficus 3X6 



D. fusiformis . . . . 3 X 5 * 

 D. ovatus . . . . .3X5 



D. polydactylus . . . . 3X5 



D. simplex also will doubtless show three radial pieces when perfect 

 examples are found ; D. cornigerus differs from other species in having 

 two brachials coming immediately from the primary radial. This 

 is the only species showing satisfactorily such an arrangement. Shu- 

 mard quotes, above, D. ovatus as having two radials repeated five 

 times, yet in his description of the species (Owen and Shumard, Geol. 

 Survey of Iowa, &c., p. 590) he says, "Several joints of the arms 

 remain attached to one of the superior plates, in the 0)ily specimen we 

 have been able to procure. The first joint is of a rectangular form, and 

 supports a cuneiform joint, on the bevelled edges of which is the com- 

 mencement of the two series of smaller plates." The formula for the 

 radial pieces will be, — 



Radial pieces . . . 1 to 3 X 5. 



Arms. But few specimens have been figured preserving the arms. 

 They generally come off in five pairs, often bifurcating below, until 

 they attain even to the number of forty divisions, as in D. polydactylus. 

 D. fusiformis has ten arms (five pairs ) without bifurcations. D. ovatus 

 has, most probably, the same number. D. polydactylus, as we have 



* Austin (Monograph of Crinoidese, pi. 5, fig. 6, c) figures a single ray as having 

 three small radials above the large primary radial. His specimen (pi. 5, fig. 6, b) 

 is very imperfect, and we think it highly improbable that four radials exist in the 

 individual there figured ; such a mistake is more easily made than mistaking Dicho- 

 crinus elongatus for a Platycrinus. Hexacrinus macrostatus of the same authors has 

 all the appearance of a Dichocrinus. {loc. cit., pi. 6, fig. 3, a.) J. Miiller in his paper 

 (Uber neue Echinodermen des Eifeler Kalkes, pi. 1, fig. 3) figures a fossil which- 

 bears a most remarkable resemblance to a Dichocrinus ; he calls it Hexacrinus. 

 VOL. V. 3 



