128 BULLETIN OF THE 



abactinal hemisome after the primary radials and dorsocentral, were 

 given by Ludwig the name of intermediate plates. These plates immedi- 

 ately following the primary radials form in the interradii, and may there- 

 fore be called interradials or basals. In the line of the radii also there are 

 new plates beginning to form, among which may be mentioned underbasals 

 and radial shields. We shall first consider the basals and interradials. 



Basals. — The first interradial plates form on the periphery of the 

 abactinal hemisome on interradii between contiguous radialia. They are 

 triangular in shape, and occupy a triangular interspace between adjoining 

 primary radials. These plates are commonly confounded with the orals, 

 which are forced to the actinal surface of the disk before the interradials 

 arise. The first set of interradial plates may be known as the abaxial 

 basals or first interradials. The next plates to form on tlie abactinal 

 hemisome after the abaxial interradials are also interradials, and arise in 

 the corners left between the dorsocentral and contiguous radialia. They 

 arise as trifid or quadrifid calcareous spicules, and are five in number. 

 They are not, however, synchronous in origin. Even in this stage, 

 although two rings of interradials are formed, the radialia are not sepa- 

 rated from the dorsocentral, and the surface of the abactinal hemisome 

 is mostly made up of the " primary rosette " of plates, the dorsocentral 

 and the radialia. As the growth of the new interradial plates proceeds, 

 the increase in size of the radialia and dorsocentral is not the same as 

 that of the interradials, and the original rosette is more or less contorted 

 in its form. Almost simultaneously with this contortion appear a number 

 of plates, among which may be mentioned a new ring of interradials, 

 five radial plates between the dorsocentral and the radialia, and the 

 radial shields. In the formation of new rings of interradials, at least in 

 the next set of plates, the newly formed seem to lie adaxially to those 

 already developed. The next ring of interradials arises between the last- 

 formed interradials and the periphery of the dorsocentral. 



In the case of the radial series of plates, however, the next plates to 

 form after the radialia are the radial shields. These plates are remark- 

 able in more than one way. Anatomically they can be recognized in all 

 genera of Ophiurans, and their mode of development in pairs is exceptional 

 among radial plates of the disk. All abaxial plates thus far formed on 

 the abactinal hemisome have originated singly, one in each interradius, 

 and are five in number. The primary radials and basals follow the same 

 law as far as the number five goes. The radial shields are the first in 

 the abactinal hemisome to originate in pairs. On the actinal side we 

 shall find all plates originate in pairs except the tori, or jaw-plates ; but on 



