MUSEUM OF COMPARATIVE ZOOLOGY. 45 



Primary Radials. — The first radials of a Crinoid, according to Sladen, 

 are represented by the Intermediare in Asterina.* There is but one set 

 of plates which can be compared with the Amphiura primary radials, 

 and these are the first of the dorsal plates of the arms. Notwithstand- 

 ing the fact that these plates have a different time of originf as regards 

 the genitals, Sladen regards it possible that they may be called the 

 primary radials. 



If we accept these plates as homologues of the primary radials, they 

 must be regarded as very much belated in time of formation. Inside 

 the radials of the disk of Asterias are representatives of the under-basals 

 and similar radial plates. 



Connectives. — The homologies of the numerous small plates and bars 

 by which the primary plates on the abactinal hemisome of the body are 

 joined together were not identified with definite plates in the Am- 



* Although it can readily be granted that the first dorsals occupy the same 

 relative position as the radials in Amphiura, it is a question whether we are justi- 

 fied in carrying our comparisons so far as tohomologize them. I grant that their 

 position on the radius is the same, and that retardation in the time of appearance 

 of plates has very little importance morphologically ; but it must be remembered 

 that we are attempting to homologize plates in stages where we know there are 

 unrepresented plates inter se in stages where there are plates in the starfish which 

 cannot be referred to those of Crinoids, and plates in the Crinoid which have, as 

 far as we know, no representatives in the starfish. 



t "While it is generally true that in Ophiurans the radialia appear before the 

 " basals," in one case, according to P. Herbert Carpenter (On the Apical System 

 of Ophiurids, p. 8), basals are formed on the disk where no radialia are repre- 

 sented (Ophiomitra exigua, Lyman). It would seem improbable that in this in- 

 stance radialia had formed and been absorbed. Carpenter (Notes on Echinoderm 

 Morphology, No. XL) says of the radialia, " They appear before the basals in the 

 Ophiurids, but after them in other groups." I confess surprise at this statement, 

 especially as three years before he had himself pointed out that Ophiomitra exigua 

 "has no radials at all, nothing but the five interradial basals (3) intervening be- 

 tween the dorsocentral (1) and the radial shields," and I can only explain it on the 

 supposition that he believed that radials had formed and disappeared, that they 

 are formed later, or that he had changed his mind on the subject. 



Carpenter's criticism of my use of " abaxial basals " I will not consider here ; 

 but as I nowhere in my paper use the combination "adaxial interradials," I fail 

 to see why he should speak of any plates as my adaxial interradials (" his adaxial 

 interradials "). 



Carpenter's statement (op. cit., p. 313, lines 12-17) that every previous writer 

 regards basals and interradials as fundamentally distinct, seems to have been 

 written without remembering the fact that Sladen in considering certain star- 

 fishes (op. cit., p. 33, lines 27, 29) uses interradial for basal, and to explain what 

 he means by interradials uses the following combination : " interradials (i. e. 

 basals)." 



