412 STUDIES IN EVOLUTION 



immature stages of higher species, and in the Terebratellidse 

 it is evident that great uncertainty must exist in the identi- 

 fication of specimens not fully adult. Now, finding that 

 Paleozoic genera of both loop and spire-bearing stocks (Ancy- 

 lobrachia and Helicopegmata) in the adolescent period like- 

 wise pass through metamorphoses representing the structures 

 of other genera and even other sub-orders, it is manifest that 

 species cannot be referred to their proper genera, nor genera 

 correctly defined, unless the individuals studied are adult and 

 their characters constant for a definite period of time. 



Development of the Loop in Dielasma turgidum. 



The earliest stage thus far observed was found in a speci- 

 men a little over four millimetres in length (Plate XXVI, 

 figure 1). The loop at this time is composed of two broad 

 descending lamellsB, which begin at the ends of the crura and 

 extend forward, curving ventrally until they unite in the 

 median line, forming an angular ridge, acuminate in front. 

 As previously mentioned, this structure is very similar to 

 that of Centronella, and this stage is therefore called the 

 centronelliform stage. 



The first change in the form of the loop is brought about 

 by a resorption of the pointed anterior portion, so that the 

 outline is re-entrant in front (figure 2). Further resorption 

 in the same manner results in the production of two poste- 

 riorly directed branches, as shown in figure 3. This form 

 may be considered as an early immature Dielasma loop, as 

 subsequent growth does not materially modify its general 

 characters. 



The adult loop, represented in figures 4-6, differs from 

 the early Dielasma stage chiefly in the divergence of the 

 descending branches. 



In the centronelliform stage the lamellae converge, and the 

 loop extends half the length of the shell. Both of these 

 relations gradually alter until, in the early Dielasma stage, 

 the descending branches are nearly parallel, the loop extends 

 less than half the length, and finally, when mature, the 



