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Transactions. 



the loop complete. For the earliest stages with a high septum and with 

 the descending portions of the loop incomplete the term " pre-Magadini- 

 form " may be used. 



Beecher has compared the pre-Magadiniform stages with the adult 

 condition of the loop in Bouchardia and Megerlina, but there are import- 

 ant differences which make the use of the terms " Bouchardiform " and 



Fig. 1. Fig. 2. 



Fig. ] . — TerebrateMa rubicunda, Chetwode Islands. Pre-Magadiniform stage, before 

 the appearance of the hood on the septum. Length of ventral valve, 3 mm. 



Fig. 2. — Terebratella rubicunda, Chetwode Islands. Pre-Magadiniform stage, with 

 small hood on septum. Length of ventral valve, 3-5 mm. 



" Megerliniform " misleading. In each of these genera, as in Magas, the 

 ascending branches do not unite to form a ring, while in B. rosea the 

 descending branches are completely absent. In the young stages of 

 TerebrateMa there is never an incomplete ring, while the growth of the 

 descending branches commences before any indication of the ascending 

 branch can be found on the septum. It is probable that in loop characters 

 B. rosea is degenerate from a stock originally possessing pre-Magadiniform 

 characters. The loops of the fossil species of Bouchardia are as yet un- 

 known, and their description will possess the highest interest. 



In the young of TerebrateMa rubicunda there is a great deal of latitude 

 in the relations between size of shells and stage of loop-development attained. 

 Generally speaking, the Magadiniform characters are attained at a length 



Fig. 



Fig. 4. 



Fig. 3. — Terebratellu rubicunda, Chetwode Islands. Pre-Magadiniform stage, with ring 

 developed from hood. Length of ventral valve, 3 - 5 mm. 



Fig. 4. — TerebrateMa rubicunda, Foveaux Strait. Late pre-Magadiniform stage, re- 

 taining a hood on the septum when the descending branches are nearly complete. 

 Length of ventral valve, 6 mm. 



of 4 mm., but one specimen of 6 mm. is still in the pre-Magadiniform stage. 

 Magelliform characters usually commence at a length of 6 mm., and are 

 quickly superseded by early Terebratelliform characters. 



The only observations I have been able to make on loop-development 

 additional to those of Fischer and Oehlert relate to the very early stages. 

 The growth of the crura commences, as they have pointed out, before any 



