MUSEUM OF COMPARATIVE ZOOLOGY. 51 



arm of the young starfish is broken from the body, always remains on 

 the arm. 



7. The dorsals, or median roio of plates on the dorsal surface of the 

 arm, originate peripherally to the first dorsal ("radial"), and are at 

 first relatively very large. 



8. The dorsolaterals do not appear in the same sequence as the dor- 

 sal. (The oldest dorsolaterals may not be the nearest to the body.) 



9. Marginals appear after ambulacrals and before (of this there is 

 some doubt) interambulacrals (adambulacrals). 



9. Oral ambulacrals are the first plates to form on the actinal hemi- 

 some. When they first appear there are the following calcifications on 

 the abactinal hemisome : 1st. 5 terminals; 2d. 5 genitals; 3d. 1 dorso- 

 central ; 4th. 30 spines on terminals. 



10. The oral ambulacrals are at first parallel with the radial ails-de- 

 sac of the water system. By subsequent growth they are placed at 

 right angles to the same. They are at first ten in number. 



11. The interbrachial ends of the oral ambulacrals of adjacent radii 

 (arms) grow towards each other, forming two parallel ends in each 

 interradius, of which each bears two spines. The median end of each 

 oral ambulacral bifurcates into a dorsal and ventral part. 



12. All other ambulacrals, with the exception of the oral, originate 

 with axes at right angles to line of radii. They form near the middle 

 line of the under side of the ray, and grow towards the peripheral. 

 The adoral are the first formed. They bifurcate in the neighborhood 

 of the median line. 



13. The first interambulacrals (adambulacrals) form after the cor- 

 responding ambulacrals in the interval between the marginal ends of 

 successive pairs of the same. Their centre of ossification is from the 

 first different from that of the ambulacrals. 



14. Marginals form before (?) interambulacrals (adambulacrals). 



15. The first interbrachials (odontophores) originate as heart-shaped, 

 interradially placed calcifications, five in number, each abactinally 

 placed to the interbrachial ends of the oral ambulacrals. In one speci- 

 men four of these were first observed ; that which lies in the same 

 interradius as the madreporic tube was retarded in growth. The first 

 formed interbrachials are not wholly concealed from view, as in deep-sea 

 Asteroids. When the first interbrachials (odontophores) form, the 

 following plates have begun to appear: 1st. 5 terminals; 2d. 5 geni- 

 tals ; 3d. 1 dorsocentral ; 4th. 10 oral ambulacrals ; 5th. 20 ambu- 

 lacrals. 



