210 THE ATLANTIC. [chap, i v. 



the Linnsean Society," OpMocoma didelphis, from its opossum- 

 like habit of carrying its young upon its back, I do not think 

 that it can properly be relegated to any genus at present de- 

 lined, but it will doubtless fall into its place when the Ophiurids 

 shall have been revised. 



The disk is about 20 mm. in diameter ; and the arms are four 

 times the diameter of the disk in length. The disk is uniform- 

 ly coarsely granulated ; the arm-shields, which are well defined 

 through the membrane, are rounded in form and roughly gran- 

 ulated like the remainder of the disk. The character which at 

 once distinguishes this species from all the others of the genus 

 is, that the normal number of the arms is six or seven instead 

 of five, which is almost universal in the class. The number of 

 arms is subject to certain variation. I have seen from six to 

 nine, but never fewer than six. The arm-spines are numerous 

 and long. The general color of the disk and arms is a dull 

 greenish brown. 



A large proportion of the mature females, if not all of them, 

 had a group of from three to ten or twelve young ones clinging 

 to the upper surface of tlie disk by their arms. The largest of 

 these were about a quarter the size of their mother; and they 

 graduated down in size until the smallest had a diameter of less 

 than 1*5 mm. across the disk. The largest and oldest of the 

 progeny were always uppermost, farthest from the disk, the 

 series decreasing in size downward, and the supply evidently 

 coming from the genital clefts beneath. In several specimens 

 which I examined, although by no means in all, there were 

 groups of eggs and of young in still earlier stages, free in the 

 body-cavity in the interbrachial spaces. 



It thus seems that in this case the true marsupium is a 

 portion of the body-cavity, and that the protection afforded by 

 it is supplemented by the attachment of the young to the sur- 

 face of the disk, maintained for some time after their extrusion 

 or escape. 



