74 F. A. BATHER 



4. With the extension of the ambulacra in length, the radials 

 grow outward, and the cross section at the ambitus becomes more 

 stellate. 



5. Concurrently the number of ambulacrals (side-plates, etc.) 

 increases. 



6. A change in the outHne of the ambulacra outside the deltoids 

 from broadly lanceolate, through narrowly lanceolate, to the 

 characteristic outline of the adult, which is broad in the proximal 

 region, then rather rapidly tapering to a region from one-third to 

 two-thirds the distance from the proximal end, then narrow and 

 gently tapering. 



7. The distal region of the ambulacrum is in all stages depressed 

 below the edges of the radial sinus, but the proximal region, which 

 in the young is flush with the sinus, gradually rises above it, until 

 in the largest individuals it projects conspicuously; and this pro- 

 jecting portion increases in length, until it nearly (though never 

 quite) attains the distal lip of the sinus. 



8. Concomitantly with the preceding change, the edges of the 

 sinus become sharply beveled, and the two beveled faces of 

 adjacent limbs of the radials meet in a ridge on the deltoid. 



9. The curve of the side of the theca below the ambitus, though 

 described above as straight or concave, is never a simple curve, 

 but consists of three parts: (a) the almost vertical sides of the 

 basal circlet, (b) the outwardly spreading lower part of the radials, 

 which is straight or feebly convex, (c) the part of the radials below 

 the lip, which in the young bends outward almost imperceptibly, 

 but gradually lies at a sharper angle to region b, until it is at last 

 horizontal. 



10. The distal parts of the radial above this region c come to 

 be marked off from the body of the radial by a vertical depression, 

 which at its upper end meets the distal end of the beveled surface 

 mentioned under (8). 



11. Various changes of proportion in details are scarcely worth 

 setting out, since all follow from the one great change, namely the 

 extension of the ambulacra outward, arching upward, and bending 

 downward, unaccompanied by equally rapid growth of the other 

 elements. 



