DEVELOPMENT OF TEREBRATALIA OBSOLETA 409 



XXV, figure 10) they show lacunae produced by resorption. 

 The structure of the loop at this time agrees with that in 

 Muhlfeldtia, and this period of development has been called 

 the muhlfeldtiform stage. 



The ends of the descending branches have continued to 

 widen on the septum (Plate XXV, figure 11), and extend 

 toward the ascending branches, with which they soon join 

 (figure 12), bringing in the terebrataliform type of structure. 

 The completion of this stage is accomplished by the further 

 separation of the ends of the ascending branches, and by the 

 resorption of the expanded ends of the descending branches 

 to form the connecting bands with the septum (Plate XXV, 

 figures 13, 14, 15). 



It should be noted that the septum in the cistelliform stage 

 is wholly anterior to the middle of the length of the dorsal 

 valve. Septal growth takes place chiefly on the posterior 

 end, and at the same time resorption along the anterior edge 

 serves to move the septum backward, until by the time the 

 terebrataliform stage is reached it is posterior to the middle, 

 and in adult specimens it is in the umbonal region. 



A comparison of the growth-stages of T. obsoleta with 

 those in Macandrevia cranium and Dallina septigera shows 

 considerable similarity, except, of course, in the adult con- 

 dition. The general features of each are alike, and may be 

 correlated in the same manner, stage for stage. The septum 

 in T. obsoleta in the platidiform stage is considerably broader 

 than in the other forms, and the descending branches join it 

 considerably lower down. The two stages preceding the 

 platidiform, which present the brachial structure first of 

 G-wynia and then of Cistella, are of chief importance. 



