292 STUDIES IN EVOLUTION 



of transformations takes place. These have been termed, by 

 Fischer and CEhlert,^ the prcemagadiform, magadiform, maga- 

 selliform, terebratelUform, and magellaniform stages, from 

 their resemblance to the loops of the genera suggesting these 

 names. The prcemagadiform stage is here divided into the 

 houohardiform and megerliniform stages. To these may be 

 added the earlier larval stages resembling Gwynia and Cis- 

 tella, as in the previous group, and showing a parallel 

 development in the first two stages. 



These two groups of the Terebratellidse usually have been 

 considered as part of the family Terebratulidse, although 

 King,^* in 1850, proposed the name Terebratellidae to include 

 Terehratella, Muhlfeldtia, and Ismenia, on account of the 

 attachment of the loop to the septum of the dorsal valve in 

 these genera. Friele " and Deslongchamps ^ next showed that 

 Macandrevia cranium and Dallina septigera passed through a 

 series of changes in which the loop was united to a septum in 

 all but the last stage. This completed loop in Macandrevia, 

 composed of two descending and ascending lamellae, was be- 

 lieved to be homologous with the loop of Terebratulina and 

 Liothyrina, and the family proposed by King fell into disuse. 

 It can now be shown, however, that the loop of Macandrevia 

 is made up of a primary portion corresponding to the entire 

 loop of Liothyrina, and a secondary j)art which has no equiv- 

 alent in the calcified lamellse of Liothyrina or Terebratulina, 

 but in them is represented in the fleshy portion of the arms, 

 as previously recognized by Hancock. ^^ 



The loop in Terebratulina is equivalent to the descending 

 lamellae in Terehratella, from the crural points down to and 

 including the bands connecting with the septum. In Magel- 

 lania and Macandrevia the connecting bands of Terebratella 

 are represented, except in old specimens, by slight projec- 

 tions from the descending branches, and in these genera, 

 therefore, the primary loop is incomplete.* The true rela- 



* The prongs or points below the ends of the crura on the primary lamellse 

 in Spirifer also represent portions of a loop. More close analogy is seen in later 

 forms of Atrypa having a disunited loop. 



