32 VERRILL 



increased by the formation of new plates between the apical terminal 

 or ocular plate and the plate next to it, the oculars being forced 

 farther away as the numbers and growth of the plates continue to 

 increase. But besides the primary rows of plates, other rows and 

 scattered ossicles may appear, to fill up the dermal spaces produced 

 by the increasing diameter of the rays and disk. In some forms of 

 starfishes this is accomplished largely by the rapid growth of the 

 primary plates, which may thus become very large, but this produces 

 a rather inflexible skeleton. In the Asteriidae flexibility seems to 

 be an essential feature, especially as most of the species cling to 

 irregular stones and similar objects, and rarely live habitually on 

 smooth mud or sand. Therefore, the skeleton has come to be com- 

 pleted in this group by the constant addition of small ossicles movably 

 articulated between the rows of larger ones, especially on the dorsal 

 surface, both longitudinally and transversely. 



These later interpolated ossicles may form regular or irregular 

 longitudinal rows between the median and marginal rows, or they 

 may be so interpolated as to form a more or less regular reticu- 

 lation ; in other cases they may be entirely irregular, with no deter- 

 minate arrangement, seeming to be introduced wherever needed at 

 the time. 



These variations in the modes of interpolation of the plates, 

 together with their forms, which may vary from linear to broad, 

 angular, lobate, and scale-like forms, combined with variations in 

 the spines, give rise to the great variety of structures and forms 

 seen in the dorsal surface of starfishes of this family. 



Moreover, in most species, the rapid increase of the dorsal 

 skeleton alone apparently does not give sufficient space for the 

 rapidly growing internal organs, especially the reproductive organs, 

 within the bases of the rays, and therefore new rows of plates (fig. 

 i, iii, iv) must be interpolated between the inferomarginals and the 

 adambulacrals, and sometimes between the upper and lower mar- 

 ginals, to increase the diameter of the rays. The most constant of 

 these rows is that which I have designated as the peractinal (pa). 

 This row appears very early in many species and often nearly 

 reaches the tip of the ray, but usually not quite to the ocular plate. 

 But the other actinal rows (subactinals or interactinals) (so) are 

 successively shorter and often do not reach the middle of the ray. 

 The plates situated proximally in these rows appear earliest, and 

 new ones are added at the distal end of each row. This is also the 

 case with the interpolated dorsal and lateral rows. In many species, 



