NOTES ON ECHINODERM MORPHOLOGY. 5 



surround the dorsocentral and form the '^ rosette " are not the 

 radials (4) as they are in Ophiomusium pulchelluni 

 (PI. I, fig. 9), or even in Ophioglypha minuta (PL I, 

 fig. 8); but they are placed interradially as in Ophioglypha 

 lap id aria (PI. 1, fig. 4), and correspond to the basals (3) 

 of a Crinoid. In describing the disc of O. minuta, Lyman 

 mentions " a central group of five primary plates whereof the 

 middle one is pentagonal, while the five surrounding it are 

 transverse oval ;" and in O. lapidaria he speaks of " a close 

 rosette, consisting of a pentagonal primary plate, surrounded 

 by five others, smaller and of an irregular shape. ^' He thus 

 takes no account in his specific descriptions of the relative 

 positions of the "primary plates," a point which might perhaps 

 be of use when other characters were obscure. The manner in 

 which these plates vary both in position and in development 

 within the limits of a single genus is very remarkable, and I 

 propose to dwell on it in some detail on account of its morpho- 

 logical bearings. 



Table showing the Variations in the Arrangement of 

 THE Apical Plates in Three Genera of Ophiurids. 



Ophiogly- 

 pha . 



Ophiomu- 

 sium . 



Ophiozona. 



RadiaU lar^e Radials partly 

 and in lateral separated 

 contact. 1 by Basals. 



convexa. 



forbesi. 



sculpta. 



scutata. 



solida. 



undata. 



lunare. 

 liitkeni. 

 pulchellum. 

 simplex. 



minuta. 



Radials com- 

 pletely 



separated hy 

 Basals. 



fasciculata. 



imbecillis. 



lacazei. 



lapidaria. 



tenera. 



flabellnm. 

 s;ranosum. 



marmorea. antillarum, 

 tessellala. depressa. 

 jinsularia. 



abyssorum. 



Under-basals, 



Basals, and 



Radials. 



scalare. 

 validum. 



clypeata. 



Central Plates 

 of Disc small, 

 and without 

 any definite 

 arrangement. 



costata. 



lepida. 



loveni. 



cancellatum 

 laqueatum. 



impressa. 



