Loop-hearing Brciehiopoda. 379 



The type is T'erehratella chiliensis Broderip, sp.= 7! dorsata Gmelin, 

 sp., from the Straits of Magellan. Fischer and CEhlert^ have 

 described in detail the development of the loop in this form. Their 

 researches also include Magellania venosa Solander, sp., which was 

 found to pass through all the stages of Terehratella dorsata, and 

 after losing the processes connecting the primary lamellae with the 

 septum finally results in adult 3fagellania. 



Magellaniinm. 



The first stage described by these authors, Plate I, figure B, 

 showed only a septum anterior to the middle of the dorsal valve.* 

 The next stage was called the prmmagadiform stage, Plate I, 

 figures C«, Da, but it may well be divided into two stages, which 

 correspond in structure to adult Bouchardia and Megerlina. The 

 houchardifortn stage, figure Qa^ has a high quadrangular septum in 

 the dorsal valve, and on the posterior distal angle, there is a small 

 circle, or calcaieous ring. The crura are present, but the primary 

 lamellae have not yet appeared. In the next stage, the inegerlini- 

 form, figures Da, Da', the ring has increased in size, and below, on 

 the septum, have appeared two projections, or points, which are the 

 beginnings of the descending primary branches. 



The subsequent, or magadiform, stage, Plate I, figure Ea, shows 

 the completion of the descending branches to form the primary 

 loop, and also the enlargement of the secondary loop, or ring. Dur- 

 ing further growth, the primary and secondary loops approach each 

 other on the septum, then coalesce and make the magaselliform con- 

 dition represented in figure Fa. 



The ventrally projecting, free portion of the septum next is 

 absorbed, and the branches of the loop become attenuated, but still 

 the descending branches remain connected with the septum, and 

 thus the terebratelliform stage is completed, Plate I, figure Ga. 



Magellania venosa, after passing through all the stages described, 

 including the terebratelliform, loses the connecting bands, and 

 develops into the final magellaniform type of structure, Plate I, 

 figure Ha. Moreover, Magellania lenticular is, M. flavescens, Tere- 

 hratella cruenta, and 7! rubicunda, as far as observed, correspond 

 closely in their development with the morphogeny of 3f. venosa. 



A fact of importance noticed by Fischer and GEhlert® is, that 

 these species are confined to the southern hemisphere. The other 



* An earlier gwyniform stage has been observed by the writer in a young example 

 of Magellania fiavescens. 



