68 



this seems to be the most usual case — 3 (fig. 1); more rarely the number is greater, up 

 to 7; but in this case the transverse bands are usually interrupted in several places, 

 and rudimentary (fig. 4). In like manner the spines attached along the calcareous ribs 

 are very differently developed. Sometimes they are proportionally small, and in great 

 numbers (fig. 4) sometimes the number is more limited, in which case they usually — 

 especially towards the sides — attain a very considerable length (almost like the exterior 

 furrow-spines). 



Also the disc itself is subject to several variations. Its form is seldom perfectly 

 circular, but often broader on one side, and irregular in a greater or less degree. Its dorsal 

 spines are likewise in different individuals more or less developed, sometimes higher and 

 more closely packed, sometimes lower and more spread. Finally the form and size of the 

 madreporic body is subject to divers variations, even in equally developed individuals. It is 

 however always naked, and never, as in the other species endecacnemos, covered with spines. 

 The color of the animal varies, as already stated, through a series of tints, from light orange 

 to deep purple red. 



b. Accidental variations. 



(Monstrosities). 



Under this head I would class 2 cases which I have observed, and in which one of 

 the arms in an otherwise normally and fully developed individual exhibited an appearance 

 very different from what is usual. 



Both cases were evidently caused by accidental mutilation, and consequent regene- 

 ration of the arm. In one instance the arm was very short, scarcely half as long as the 

 others, but nevertheless in its basal section quite as much developed as the others, with 

 swelling organs of generation, and well developed calcareous ribs and spines; while the ex- 

 terior part of the arm was very slightly developed, and sharply distinguished from the rest 

 of the arm. being in the form of a little thin lash-like appendage with rudimentary spines 

 and water- feet. The arm had evidently been quite lately broken off at this place; and 

 from the wounded surface, there had sprouted a nascent new arm to replace the lost part. 

 I have also observed a perfectly similar instance in the Brisinga endecacnemos (see Tab. VII, 



fig. 1 a). 



In the second case the arm was of the usual length, but highly remarkable for 

 having the extremity bifurcate, or going out in 2 nearly equally long branches completely 

 normally formed with all appertaining parts, and having at each end the terminal organ of 

 sense (Tab. II, fig. 3). The ambulacral furrow running along the arm divided itself, at the 

 issue of these two branches, precisely in the same manner as in a Pentacrinus or an An- 

 tedon; and the ambulacral skeleton seemed likewise to have been similarly affected. That 



