394 C. E. Beecher — Development of Terehratalia ohsoleta, Ball. 



secondaiy loop are not present in all specimens, and do not seem to 

 be important characters. The cirrated band at this time, Plate II, 

 figures 4, 5, extends continuous!}^ from behind the mouth to the 

 origin of the descending branches, and along them to the septum, 

 thence obliquely backward on the septum, and over the ascending 

 branches to the median line, where new cirri are introduced. 



The ascending lamellae from the septum already have begun to 

 divide or separate anteriorly, and in the next stage, Plate III, 

 figure 10, they show lacunae produced by resorption. The structure 

 of the loop at this time agrees with that in Milhlfeldtia, and this 

 period of development has been called the 7niXhlfeldtiforwt stage. 



The ends of the descending branches have continued to widen on 

 the septum, Plate III, figure 11, and extend toward the ascending 

 branches, with which they soon join, figure 12, bringing in the 

 terehrataliforin 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 descend- 

 ing branches to form the connecting bands with the septum, Plate 

 III, figures 13, 14, 15. 



It should be noted that the septum in the cistelliforni 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 backwards, until by the time the terehratalifor'm stage is 

 reached it is posterior to the middle, and in adult specimeils it is in 

 the umbonal region. 



A comparison of the growth stages of T. ohsoleta with those in 

 Macandrevia cranium and Ballina septigera shows considerable 

 similarity, except, of course, in the adult condition. The general 

 features of each are alike, and may be correlated in the same man- 

 ner, stage for stage. The septum in T. ohsoleta in the platidiforin 

 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 struc- 

 ture first of Gwynia and then of Cistella are of chief importance. 



Yale Museum, New Haven, Coun., March 15th, 1893. 



