Growth-changes in Brittle Stars. 105 



or by the failure of the median ray to develop on the regenerating 

 side of a normal 3-rayed individual. But only about one specimen 

 in ten has 5 rays. 



3. In fission the dividing plane does not cut through a jaw, but passes on 



one side or the other; accordingly a half-disk may have 2, 3, or 4 

 jaws; normally there are 3. 



4. The disk-covering, when the adult form is first assumed, consists of 



the typical primary-plates, shown by Ludwig to be characteristic 

 of Ophiactis, but owing to repeated fission, the covering of the disk 

 is ultimately made up (aside from the large radial shields) of small 

 plates with no regular arrangement. 



5. The torus bears at first none, then i, 2, 3, 4, 5, and sometimes 6 teeth, 



of which the oldest (first-formed) is uppermost and the youngest 

 is lowest (most ventral). 



6. Young vertebrae are very much longer than high or wide; the alas are 



the first outgrowths to appear, soon followed by the aboral hypa- 

 pophyses; the zygosphene, protapophysis, and epanapophysis 

 appear almost immediately thereafter. 



7. Typical vertebrae occur in the adult arm from about the eighth to the 



twentieth segment. 



8. Of the plates covering the vertebrae, the side arm-plates appear first, 



followed by the under arm-plates and lastly by the upper. The 

 side arm-plates, of any one segment, are broadly in contact v.ith 

 each other both above and below on the young joints, but are ulti- 

 mately separated dorsally by the growth of the upper arm-plates 

 and ventrally by the lower. 



9. The lowest arm-spine is the first formed; all succeeding spines arise 



serially dorsal to it. 



10. The typical number of arm-spines is to be found on about the tenth 



segment of an adult arm and thence distally for a varying number 

 of segments, rarely as many as 40. 



11. The lowest arm-spine has no characteristic form. 



12. The tentacle-scales appear with the first under arm-plate; they may 



not be homologous with the arm-spines. 



Amphipholis squamata (Delle Chiaje). 

 The peculiar habitat of the specimens of Amphipholis collected at 

 Montego Bay led me to question whether the species was really identical 

 with that found on shelly bottoms in New England waters. But critical 

 comparison of these Jamaican specimens with others from Woods Hole 

 (Massachusetts), Casco Bay (Maine), Grand Manan (New Brunswick), 

 Norway, and Naples has forced me to the conclusion that they can not 

 be distinguished and are undoubtedly entitled to the name squamata. At 

 Montego Bay, the species was found only in the red sponge in which Ophi- 

 actis occurred so abundantly, but in that situation it was fairly common. 

 The smallest free-living specimen seen had the disk a millimeter across and 

 of an orange-red color, a peculiarity of the young noted by Fewkes. Most 

 of the specimens with disks more than 2 mm. across contained young in a 

 more or less advanced stage of development. The youngest embryo 

 examined had the disk 0.35 mm. across, and the arms, each with 3 segments, 

 projected only 0.20 mm. beyond the disk margin. The largest adult did 

 not exceed 3 mm. in diameter. 



