MUSEUM OF COMPARATIVE ZOOLOGY. 243 



although brilliant in life, fade to a dull straw-color in alcohol, while others 

 present a mottled and varied look without any special pattern. 



Beginning with northern waters, we find, on the shores of Denmark and 

 Sweden, a type which is without question the Asterias fragilis of Abildgaard. 

 It is large, and has a fleshy disk swelling into lobes in the interbrachial 

 spaces; a diameter of 15 or 10 mm. is a common size, and specimens with a 

 disk of 9 mm. have still young characters. Here it may be observed that 

 the upper arm-plates, at the base of the arm, furnish the best test of the ani- 

 mal having taken on its adult form. For example, in the group under dis- 

 cussion, these plates have a length to breadth about as 2:3; whenever, 

 therefore, the length of these plates, near the disk, is greater than, or equal 

 to, the breadth, it is safe to assume that the creature is still in its young 

 stage. The arm-spines of 0. fragilis vary more than those of any other 

 European type ; they may be found very short, and so thickened that the 

 edge thorns, of which there are only 14, are nearly obliterated; or quite 

 long, flattened, and glassy, with as many as 22 well-marked thorns on each 

 edge. Often, or perhaps generally, the upper arm-plates, instead of being 

 decidedly angular, have the outer side cleanly curved. The radial shields 

 are somewhat sunken in the swelled disk, and are a little bent near their 

 small end. The armature of the disk consists of a great variety of short, 

 stout, stumps, accompanied or not by a few slender cylindrical spines (Plate 

 II., Fig. 42). A very common form of stump is with a crown of four thorns, 

 either cylindrical or else spread like a partly opened fan (Fig. 38) ; some, too, 

 may have only three thorns, whereof the middle one may be flattened and 

 elongated (Fig. 39). There may be near the centre of the disk a quantity of 

 conical grains covered with minute points (Fig. 43), with which the preceding 

 shapes mingle, especially near the edge. In the young we get the simpler 

 shapes from which are produced the full-grown stumps. Thus, a little one 

 having a disk only 5.5 mm. in diameter exhibited minute, very stout stumps 

 which were simply clavate, or else had a crown of three strong thorns (Fig. 41). 

 "When the disk was increased to 7 mm. the same shapes presented themselves, 

 but more elongated (Figs. 37, 40); and there were added a few true spines 

 (Fig. 4 2), slender, cylindrical, and with very feeble thorns at the tip and along 

 flic sides. The same shapes again occurred on a larger disk, 9 mm., with an 

 increased number of spines; and, near the edge, a much elongated modifica- 

 tion of the clavate stump (Fig. 44). 



OpJiiolhrix Rammelsbergii, Midi, and Trosch., is a half-grown specimen 

 of 0. fragilis, in which the disk stumps were encased in thick wrappers of 

 skin, giving them the look of smooth papilla;. It is a variety always to he 

 borne in mind while studying this genus, and is illustrated in Figs. 10, 11 ; in 

 one of which the skin-bag is left on, and in the other partly torn off. 



I could not find in 0. alba Grube any characters to distinguish it from a 



