138 BULLETIN QTi' THE 



Torus angularis and Teeth. — These structures arise later than the first 

 pair of adambulacral plates, but develop quite early in the growth of the 

 young Amphiura. They form as independent calcifications. The torus 

 is at first an elongated plate or bar, which later becomes semicircular. In 

 Fig. 20 this plate is represented as perforated and reticulated. It ap- 

 pears that the teeth are not separate centres of calcification, but grow out 

 directly from the adoral region of the torus. The calcareous deposit 

 which enters into their formation has the form at first of a reticulated 

 perforated triangular plate. 



B. PLATES OF THE AEMS. 



The plates of the arms are the following : — 



1. Terminals, tp. 



2. Side plates, Ip. 



3. Ventrals, v. 



4. Dorsals, d. 



Terminals. — The terminal plates (tp) which lie at the aboral extrem- 

 ity of the arms have been described by many observers. These plates 

 have been confounded by many writers with the radials. Ludwig * first 

 distinguished them from what are already described as the radialia or 

 primary radials. The fact that certain plates early formed in the 

 young Ophiurau were pushed out by a growth of new plates was known, 

 hut in all cases these plates were regarded as " primary " plates and were 

 in some instances morphologically misinterpreted. In Amphiura Krohnf 

 has figured the terminal plate, but gives it no special description. Max 

 Schultze X recognized the fact that the terminals belong to the arm proper, 

 and correctly designates them as the " erste Anlage der Arme." A. 

 Agassiz § observed the terminal plates in an Ophiupan, probably Ophi- 

 opholis, which he referred to Amphiura. He regards them as the first 



of Amphiura, it might be supposed that he does not consider these early formed 

 plates as primary plates of the test. In strict language they are mouth-plates, but 

 are so early formed in the larva that they may be regarded as primary plates. As a 

 question of opinion I think they ought to be mentioned in a discussion of the homol- 

 ogy of the odontophore. 



♦ Op. cit., p. 187. 



t Op. cit., PI. XIV. Fig. 1, J ; p. 342. 



t Op. cit., PI. I. Figs. 4, 5, c. In Fig. 6 they are lettered /. He does not seem 

 to have connected Plate c. Figs. 4, 5, with /, Fig. 6. 



^ Op. cit. , p. 20, Fig. 32, y. 



