DEVELOPMENT OF TEREBRATALIA OBSOLETA 407 



consider it as a distinct species, T. obsoleta (Plate XXIV, 

 figures 6-9), and this determination is here adopted. A 

 comparison of the two forms shows that T. occidentalis is 

 much wider and less convex, the plications stronger, and the 

 valves heavier. In the previous paper it was stated that 

 Terebratalia obsoleta was morphically equivalent to Terebra- 

 tella dorsata, and that, during growth, both went through 

 different series of transformations to reach this final equiva- 

 lent type of structure. It is now proposed to illustrate and 

 describe in greater detail these metamorphoses of T. obsoleta. 

 The generic homologies and differences are discussed in the 

 paper already mentioned. 



The earliest stage observed in this species (Plate XXIV, 

 figure 10) has a length of .3 mm. It is comparable to an 

 early stage of Cistella neapolitana described by Kovalevski.* 

 Both agree in having an incomplete circlet of centripetal 

 tentacles. The smaller and younger tentacles are near the 

 front margin, while those first formed are on each side of 

 the mouth. This condition agrees with the tentacular multi- 

 plication described by Brooks in G-lottidia, by Kovalevski 

 in Cistella and Lacazella, and by Morse in Terebratulina. 

 Tentacles 1, ti' were the first pair to be formed, and te, 

 f2', 3, 3', M, t\' followed in pairs in the order named; f5 

 has just appeared, and the corresponding one on the other 

 side is not yet seen. Figure 10 also shows the tooth-like 

 projections (cZs), forming, with the cardinal angles, sockets for 

 the reception of the teeth in the ventral valve. The adductor 

 muscles are indicated at at?, and the diductors at did; the 

 mouth is at w, with the visceral mass posterior to it. No 

 diverticula have yet appeared to form the pallial sinuses. 



When the shell has reached a length of .65 mm. (Plate 

 XXIV, figure 11), the circlet of tentacles is complete, and 

 the pallial sinuses appear as two slightly branched tubes 

 extending anteriorly from the sides of the visceral mass. 

 From the correspondence of the structure of Gwynia to this 



* See C. E. Beecher, Development of the Brachiopoda, pt. ii, figs. 15, 16. 

 Amer. Jour. Sci. t XLIV, 1892. 



