Thomson. — Recent Brachiopoda of New Zealand. 



407 



sign of the ring appears on the septum (fig. 1). I have observed several 

 specimens in a succeeding stage in which a very small hood, not a ring, 

 appears on the posterior end of the septum (fig. 2). The base of this hood 

 later becomes resorbed, and it passes into a ring (fig. 3), but sometimes 

 it persists to a fair size (fig. 4). In the large specimen of 6 mm. previously 

 mentioned, the front parts of the descending branches have already appeared 

 on the septum before the hood has developed into a ring. 



Fischer and Oehlert made no observations on the development of the 

 hinge-plate, which is a characteristic of the Terebratella-Magellania stock. 

 In their figures of T. dorsata the septum does not extend back as far as the 

 hinge-plate until the Magelliform stage is reached, whereas in T. rubicunda 

 it passes under the anterior end of the hinge-plate in the Magadiniform, 

 and exceptionally in the pre-Magadiniform. stage. The development of 

 the hinge-plate commences independently of the septum. In very young 

 specimens it starts as two almost vertical plates on the inner sides of the 

 anterior end of the socket-ridges, separated in front from the socket-ridges 

 by a hollow, but behind coalescing with them. Possibly these two plates 



Fig. 



Fig. 6. 

 Early Magadiniform stage 



Fig. o. — ■Terebratella rubicunda, Foveaux Strait 



of ventral valve, 4 mm. 

 Fig. 6. — Terebratella rubicunda, Foveaux Strait. Magelliform stage 



valve, 8 mm. 



Length 

 Length of ventral 



are united along the middle line of the valve, but, if so, they are closely 

 applied to the bottom of the valve. In succeeding stages these plates 

 descend from the socket-ridges more and more obliquely, thus gradually 

 approaching one another, while the hollow between them and the floor of 

 the valve gradually increases in size. In later Magadiniform stages, where 

 the septum now reaches back to the region of the hinge, these plates unite 

 above the septum, but farther back the}' are still applied to the bottom of 

 the valve. Finally, in the Magelliform stages 

 the septum reaches right back under the hinge- 

 plates to the umbo, and the characteristic 

 Terebratelliform hinge characters are attained. 

 The cardinal process appears at the umbo in 

 very early stages in much the same form as it 

 attains at maturity, while the hinge-teeth of 

 the ventral valve are also well developed in 

 the early stages. 



The ridge separating the muscular mark- 

 ings of the ventral valve, which is situated 

 anteriorly in Bouchardia and Magadina, but 

 posteriorly in Terebratella and Magellania, is 

 well developed in the young stages of T. rubicunda, and is situated 

 anteriorlv in pre-Magadiniform stages, centra ly in the Magadiniform 

 stages, and posteriorly in the higher stages. 



Fig. 7. — Terebratella rubicunda, 

 WellingtonHfibour. Adu]'. 

 Lergth of ventral valve, 

 1 9 mm. 



