MUSEUM OF COMPARATIVE ZOOLOGY. 129 



the abactinal side of the disk the plates which ultimately form radial 

 shields are the first bodies to arise in this way. The radial shields lie 

 just abaxially to the radialia, one on each side of a radius. In one in- 

 stance, and that in the preparation figured, a median radial plate abax- 

 ially placed to the radialia had formed before tlie radial shields began 

 to appear. In other specimens that was not observed to be the normal 

 method of formation. I have figured this specimen from its similarity to 

 a larva figured by Lyman,* about which more will be said later when 

 considering the homology of the radial shields. It is not until the radial 

 shields have reached some size that a radial plate is formed between each 

 of the radialia and the dorsocentral on the lines of the radii. This last- 

 mentioned plate, the underbasal,t tends to separate the primary radialia 

 and dorsocentral, and from the first their edges do not join each other. 



The interpolation of new plates in the abactinal hemisome now be- 

 comes more or less irregular. If any law for the formation of these plates 

 exists, the modification in size of the plates renders it extremely difficult 

 to trace it in their formation at this time. 



The nomenclature of the intermediate plates on the abactinal hemi- 

 some is similar to that given by P. H. Carpenter in his discussion of 

 Crinoidal and Ophiuran morphology.^ It seems to me, however, that it 

 would have been better, in considering the relationship of the basals in 

 the young Amphiura and their homologues in the projection of the calyx 

 of an Antedon larva, for Carpenter to have introduced, instead of his Fig. 

 II. (a copy of one of Lud wig's figures), a copy of another (Fig. 25) of the 

 same author. In this figure (Fig. 25) there is but one circle of interradial, 

 intermediate plates and no " underbasals," while the intermediate plates 

 of Ludwig's Fig. 25 and Carpenter's diagram of the Antedon (Fig. III.) 

 numerically correspond. The additional intermediate plates, both radial 

 ("underbasals") and interradial ("basals"), in the figure by Ludwig, 

 which he (Carpenter) has chosen for Fig. II., lead to a difficulty in one 

 particular in a comparison of the young Amphiura and Antedon. It is an 

 important thing to know whether the intermediate plates of Ludwig's Fig. 

 25 are the same as the Plates 3 of Carpenter's diagram {loc. cit., Fig. IL). 



* Challenger Ophiuroidea, PI. XL. Fig. 11, p. 157. 



+ Notes on Echinoderm Morphology, No. V. On the Homology of the Apical 

 System, with some Remarks upon the Blood-vessels. Quart. Jourti. Micros. Sci., Vol. 

 XXIL The homology of this plate with the Crinoidal underbasal is recognized by 

 Carpenter. 



X Notes on Echinoderm Morphology, No. V. pp. 10, 11. Quart. Journ. Micros. 

 Sci., Vol. XX. 



VOL. XIII. — NO. 4. 9 



