22 VERRILL 



same time. The first median dorsal plate of the rays appears very 

 early in the young, and the first marginals and adambulacrals soon 

 after. 



In many genera and families, but not in all, additional rows of 

 plates may be interpolated between the marginal rows proximally, 

 and are called intermarginals ; or between the inferomarginals and 

 the adambulacrals, when they are called interactinals, intermediate 

 actinals, or simply actinal plates. 1 These do not appear very early in 

 the young, and are often without spines. 



These interactinal and intermarginal plates do not belong to the 

 primary system of plates, nor do they commonly reach the apical 

 plate. Their new plates develop mostly at the tips of the rows, as 

 the starfish grows larger, and new rows may be interpolated till they 

 sometimes become very numerous, in order to increase the size of 

 the rays. 



The dorsal skeleton of the disk, in a five-rayed species, primarily 

 has five basal radials (or first dorsals), five genitals, and five pairs 

 of interradial plates, besides the plates that later become the apical 

 or ocular plates, and the centro-dorsal plate. By the interpolation of 

 new ossicles and plates in various ways, the structure often becomes 

 very complex. 



On the rays we can usually distinguish a median or carinal row, 

 extending from the basal radial to the apical plate. Other regular 

 rows may develop each side of this (the dorso-laterals), or the whole 

 surface may become covered with a tesselated arrangement, or a 

 reticulated system of plates and transverse ossicles. 2 



The dorsal plates, like the marginals and interactinals, commonly 

 bear spines or small spinules, but they may be covered with granules, 

 or with a smooth soft integument, or even appear quite naked, being 

 then covered only with a thin membrane. 



These plates and their armatures of spinules take several special 

 names, according to their forms and structure, and are often char- 

 acteristic of special genera and families and higher groups. 



When they become columnar or of hour-glass shape, and have the 

 summit covered with a radiating cluster of small slender spinules, 



1 For more details of the rows of plates and their sequence, see below, 

 under the family Asteriidae, and text-figure, i, i-iv. For the sequence in the 

 early stages of the development of the primary plates, see J. W. Fewkes : 

 On the Development of the Calcareous Plates of Asterias, 1888. 



*For more details of the skeletal plates, see the discussions under the sev- 

 eral orders and families below, especially under Forcipulata and Phaner- 



