70 CALAMOCRINUS DIOMED.E. 



basals and the dorsocentral as representing the infrabasals. When, how- 

 ever, we remember that in one and the same genus in Ophiurans we 

 may have infrabasals present or absent, and that in closely allied Star- 

 fishes there may be two or even three rings of plates developed in addi- 

 tion to the ring of plates called infrabasals by Sladen, it looks as*if he 

 and Carpenter had carried their homologies too far. I will grant that we 

 have rings of plates between the basals and the dorsocentral, the first of 

 which next to the basals may be called the infrabasal ring ; but what 

 shall we call the others ? Is it not more natural to look upon the succes- 

 sive rings of plates found in Ophiurans, Starfishes, and Echini, not as 

 strictly homologous, but as representing an earlier echinodermal stage ? 

 For in the Cystideans, Holothui'ians, and in some Crinoids, where the 

 ])lates of the test are merely so many rings of plates following one another 

 next to the dorsocentral, we cannot compare in greater detail the plates 

 of the second or third ring to the single ring of plates corresponding to 

 them in the monocyclic Crinoids. 



As Carpenter himself says, " The manner in which these plates vary, 

 both in position and in development, within the limits of a single genus, 

 is very remarkable." In the species of Ophiomusium he names, the plates 

 agree fairly well, while in Ophiozona there is a great variation. One of 

 the species described by Ludwig* shows perhaps better than any other 

 that the successive cycles of plates soon become irregular, and have no 

 special meaning. 



Furthermore, it should not be forgotten that the homologies of the pri- 

 mary plates are greatly confused from the fact that the corresponding 

 plates in the diiferent orders of Echinoderms do not appear at the same 

 period of growth in each, and that the same differences in time exist even 

 in species of the same genus. 



The central system, if I may call the stems of the Crinoids or their 

 homologues by that name, is in the Crinoids partly separated from the 

 basals by one ring of radially placed infrabasals. In the Ophiurans and 

 Starfishes the central plate may be separated from the basals by a number 

 of such rings, forming the upper surface of the disk, which with the central 

 plate correspond to the central system. The plates covering the central 

 system are developed horizontally only in the Echini, Starfishes, and Ophi- 



* See his figure of Ophioglypha maculata, Plate VI. Fig. 12, Echinod. d. Beiiiigsmeeies, Zool. 

 Jahibiichev, I. 



