6o 



MEMOIRS OF THE STATE MUSEUM 



their junction not a simple angle, but a miniature saddle, from the posterior 

 extremity of which extends straight backward a little spiniform process. 

 The number of coils in these spires varies with the age of the shell. In 

 preparing a series to show the development of these structures, it appears 

 that the shelly ribbons composing the spirals not only make fewer coils in 

 early life, but that these are of exceeding tenuity in the primary stages of 

 development. The accompanying figures show the extremes of develop- 

 ment noticed in these respects, figures i and 2 representing the character of 

 the supports in the mature condition, and figure 3, the spirals as developed 

 in an individual having a length of 2.5 mm., where the ribbon makes but 



Fir. I. |.ig. 2. Fig. 3. 



Figs, i, 2, 3. Development of Internal Apparatus in Retzia Evax. 



two revolutions. As far as can be ascertained, the loop undergoes no 

 essential modification in these early stages, though its precise character 

 in the example from which this drawing has been made, was not determined, 

 but has been drawn in. The same arrangement, however, has been seen in 

 an individual of but slightly larger growth. 



The growth of these spirals consists, primarily, in the addition to the 

 number of coils, and, secondarily, in the thickening of the ribbon. In the 

 first case, the increase in number must take place by addition to the apices 

 of the coils, and, therefore, the embryonic or primary coils of the ribbon 

 must be wholly concealed by later depositions upon them, both in length 

 and width. The apparent looseness of the coils in their primary condition 



