374 p. HERBERT CARPENTER. 



ring of the embryo Crinoid surrounds this tube near its 

 proximal or anterior end, while the central disc of the em- 

 bryo Starfish or Sea Urchin lies over the centre of its distal 

 or hinder end, at some little distance from its anterior ex- 

 tremity. The fact that the basals of the Pentacrinoid rest 

 iipon the uppermost stem-segment (fig. i) does not appear to 

 me to be a sufficient reason for considering this segment as 

 homologous with the central abactinal plate, with which the 

 homologues of the Crinoidal basals are in contact in the 

 other Echinoderms (figs, ii, vii). 



Beyrich^ has shown that in Encriiius there is an inner 

 or second circletof basals intervening between the basals (fig. 

 IV, 3, 3) and the top stem segment, and I have found (as I 

 shall shortly describe elsewhere) that these secondary basals 

 are represented in some species of Pentacrinus, but not in 

 others. According to the reasoning followed by Agassiz, 

 tempting as it seems at first sight, Ave should have to regard 

 these five plates in Encrinus and in P. hriareus as collectively 

 homologous with the central abactinal plate of the other Echi- 

 noderms (figs. ii,vii,l), because the true basals =interradials, 

 Ag.,rest upon them. But in P. asteria, in which they are ab- 

 sent, this plate would be represented by the top stem segment, 

 as in Comaiula. This, then, is another objection to our 

 considering as Agassiz' view with regard to Comatula involves 

 our doing, that the top segment of the Crinoidal stem 

 represents the central abactinal plate of the other Echino- 

 derms, and hence the question arises, What is its honiologue 

 in the Crinoids ? 



In the first place, I believe that the second or under basals 

 of Encrinus (fig. iv, '2, 2), Pentacrinus, and of the Paleeozoic 

 Crinoids (fig. vi, 2, 2), and also the interradials of the latter 

 (fig. VI, i i) are additional elements which occur in the apical 

 system of some Crinoids, while they are unrepresented in 

 other members of the Order, and in the other Echinoderms. 

 Further, I regard all the annular segments of the Crinoidal 

 stem in the same light, and believe that the central plate of 

 the apical system of the other Echinoderms is represented 

 in the Crinoids by the discoidal plate developed close to the 

 posterior extremity of the embryo (fig. viii, 1), that forms 

 the basis of the expanded disc at the base of the stem by 

 which the larva attaches itself to foreign bodies (fig. ix, 1). 

 In many of the pedunculate Crinoids the stem is attached 

 by a more or less spreading base of this kind. This condi- 

 tion was common in the Apiocrinidce and Cyathocrinidce, 



1 "Ueberdie Crinoideen des Musclielkalks," ' Abhandlun£reu der Berlin 

 Akademie,' 1857, p. 13. 



