FAMILIES OF LOOP-BEARING BRACHIOPODA 295 



their brachial supports these all approximate early stages of 

 the higher genera Magellania and Terehratella. They must 

 be regarded as arrested and degraded forms. 



The brachial supports in Kraussina and Bouchardia are 

 merely portions of the ascending branches, or secondary loop, 

 on the septum, without any traces of the descending branches, 

 or primary lamellae. These genera may be compared with 

 the houchardiform stage of Terehratella dorsata. One grade 

 higher is exhibited in Megerlina (type M. Lamarckiana David- 

 son) in which there is added to the Kraussina structure two 

 processes apparently homologous with the points belonging 

 to the descending branches appearing on the septum in the 

 megerliniforin stage of T. dorsata. These atavistic genera 

 are all austral in their distribution, but not strictly polar, 

 occurring as they do off the coasts of South Africa, Brazil, 

 Australia, St. Paul's Island, etc. 



In reviewing this group of genera, it is seen that the high- 

 est member of the series is Magellania, which reaches its 

 maximum development in size and number of species in 

 antarctic seas. The next genus below, Terehratella, ranges 

 still further toward the equator, while the atavistic types 

 Kraussina, Megerlina, and Bouchardia do not occur in polar 

 regions, but are nevertheless austral in their distribution. 



Dallinince. 

 The northern hemisphere furnishes a series of genera and 

 species, which, passing through a different and distinct series 

 of loop metamorphoses, attains in the higher members the 

 same result as those of the southern fauna, constituting a 

 case of exact parallel development. Thus the northern Ma- 

 candrevia cranium, Dallina septigera, D. Raphaelis, D. Grrayi, 

 Terehratalia transversa, T. coreanica, T. spitzhergensis, and 

 T. frontalis are very similar in the adult characters of the 

 loop to the southern 3Iagellania venosa, M. kerguelenensis, 

 M. Wyvillii, M. flavescens, M. lenticularis, Terehratella dor- 

 sata, T. cruenta, and T. ruhictinda. It is onl}^ when their 

 development is examined that a difference is manifest. 



