C. E. Beecher — De'^elopment of Terebr alalia obsoleta, Dall. 393 



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 ten- 

 tacular multiplication described by Brooks in GloWdia, by Kovalev- 

 ski in Cistella and Lacazella, and by Morse in Terebr atulina. Ten- 

 tacles ^1, ^1', were the first pair to be formed, and ^2, tl\ tS, tS', tA, 

 t4:', followed ill pairs in the order named; to has just appeared, and 

 the corresponding one on the other side is not yet seen. Figure 10 

 also shows the tooth-like projections, ds, forming, with the cardinal 

 angles, sockets for the reception of the teeth in the ventral valve. 

 The adductor muscles are indicated at ad, and the diductors at did/ 

 the mouth is at m, with the visceral mass posterior to it. No diver- 

 ticula have yet appeared to form the pallial sinuses. 



When the shell has reached a length of .65™™, Plate II, 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 

 Givynia to this stage of Terehratalia and other genera of the Tere- 

 bratellidse, it is called the gwyniform stage. 



By the time a length of 1™™ is attained, Plate II, figure 12, and 

 Plate III, figure 1, a triangular septum is visible anterior to the 

 middle of the dorsal valve. Its elevation inflects the circlet of ten- 

 tacles, making the lophophore reniform or bilobed, and producing a 

 brachial structure similar to that in Cistella. This stage is therefore 

 called the Gistelliforrn stage. The only advance over this condition 

 shown in adult Cistella is, that the band supporting the cirri is calci- 

 fied and attached to the crural points. In Terehratalia, during the 

 growth immediately following and still included in the cistelUforni 

 stage, the septum increases in height, and often two small lateral 

 expansions appear on the posterior sloping edge, figures 3, 4. 



In the transformation to the next, or platidiform, stage, figures 

 5-8, the crural points appear, and there is a narrow groove along the 

 top of the septum, which is arched over at the posterior end, form- 

 ing at this point a small cylinder, nearly vertical to the floor of the 

 valve. This is the beginning of the ascending branches, or second- 

 ary loop. The septum is quadrangular and develops on its anterior 

 edge several spinous processes, figure 7. The growth of the 

 descending branches from the crura and their union with the sep- 

 tum bring about the platidiform stage, as represented in Plate III, 

 figure 8; and the growth of the ascending branches produces the 

 ismeniform structure, figure 9. The expansions on the sides of the 



