214 VERRILL 



seasonal changes in the temperature of the water, or to the partial 

 suspension of growth during the breeding season, and the lack of 

 nourishment in the case of the female, while incubating the eggs 

 and young. Moreover, I have good reason to believe that in some 

 species there may be marked sexual differences, though my material 

 is not sufficient to determine this with certainty. 



It is known that in many species the female, while carrying the 

 eggs and young, under the mouth and oral part of the disk, arches 

 up the abactinal side of the disk to a considerable extent, and makes 

 a central cavity beneath, in which the eggs are held. (See Fisher's 

 figure, pi. LXVIII, fig. i.) In some species this seems to be only a 

 temporary condition, disappearing at other periods. But in H. tumida 

 V., of Alaska, it seems to have become a permanent condition, in an 

 exaggerated form, for the oral region and mouth are deeply with- 

 drawn and a number of the oral plates have become modified, to fit 

 this condition. Whether all such individuals are females I do not 

 know. The male may, perhaps, be quite different in form. 



Variations in the slenderness of the rays seem to be of little 

 importance in this genus, unless of great extent. The thickness of 

 the rays at base depends partly on the number of rows and extent 

 of rows of the interactinal and intermarginal plates, but also upon 

 the number and size of the abactinal ossicles. These are constantly 

 increasing in number by the interpolation of new ones, and probably 

 by the splitting up of the older and larger ones, while at the same 

 time the smaller ones are growing larger. Where such large num- 

 bers of ossicles exist, a slight change in the size or form of each will 

 cause great changes in the whole skeletal structure. 



That variations in growth, more or less periodically, may account 

 for many apparent varieties, has been notably confirmed, for me, by 

 finding certain specimens of H. sangmnolenta, with rays equal in 

 size and length, but having the marginal and other ossicles and 

 spinules as different, on some of the rays compared with others, as 

 they usually are in different " varieties " of the species, not only in 

 size, but also in the form and number of spinules upon them. The 

 variations, as between adjacent rays, may be as much as fifty per 

 cent in the size of inferomarginal plates and number of their spinules. 

 These are not cases of restorations after injury, which often also, 

 cause peculiar abnormal variations. 



The habit of carrying the eggs and young till well developed, as in 

 Leptasterias and Solaster, is conducive to variations perpetuated by 

 isolation of colonies, due to their slow rates of diffusion. The 



