528 Forty-fifth Report on the State Museum. 



Among the spire-bearing forms it has been shown that in the 

 young of Rlmjnchospira (Homoeospira) evax the ribbon is very 

 slender and makes but a few and very lax revolutions. The 

 same fact is evident in the young of Meristina Maria (as shown 

 in the accompanying figures), w^here the revolutions are few and 

 the spiral cones depressed. The growth of these solid bodies and 

 the increase in number of the revolutions could be effected only 

 by constant resorption and redeposition of the spicular calcareous 

 secretions from the brachia. 



Kovalevski, Friele, Deslongchamps, (Ehlert, Beecher and 

 others have given accounts of the development of the brachia in 

 Cistella and some of the higher genera of the Terebratellidce (Tere- 



BRATELLA, MAGELLANIA, MACANDREVIA, DaLLINA, TeREBRATALIA). 



In the early stages of such shells the brachia have the form of a 

 circlet, extending forward from about the mouth, bearing a few 

 tentacles or cirri, which are directed inwards. This condition is 

 comparable to the adult condition in Gwynia and has therefore 

 been termed the gwyniform stage. By the development of a 

 median septum in a succeeding stage of growth this circlet 

 becomes inflected medially and the resultant condition is that of 

 the adult Cistella (eistelliform stage). With the further growth 

 of the septum and the development of its lateral processes the 

 brachia gradually take on the form of the mature Platldia 

 (platidiform stage), the cirri finally becoming everted, and the 

 median unpaired arm resulting from the continued growth of the 

 basal cartilage and the multiplication of the cirri at the distal 

 extremities of the brachia. 



These developmental stages are still more clearly exemplified 

 in the formation of the loop or calcareous supports. Among the 

 TerebrateUidcB there is one group of boreal forms (Dallininm) 

 and one also of austral forms {Magellan J n<v). all agreeing in 

 having the loop, in its final condition, composed of long descending 

 and ascending branches in no connection with the median septum. 

 In the boreal forms (Macandrevia, Dallina) the development of 

 the loop has passed through a series of stages corresponding succea 

 sively with the adult condition in Platidia, Ismenia, Muhlfbldtia 

 and Terebratalta, while in the austral group (Magellania) the 

 preliminary stages are reproduced in the adult Kraussina, Botj- 



80 



