Brachial Supports in Dielasma and Zygospira. 73 



of both loop and spire-bearing stocks (Ancylobrachia and Heli- 

 copegmata) in the adolescent period likewise pass through 

 metamorphoses representing the structures of other genera and 

 even other suborders, it is manifest that species cannot be re- 

 ferred 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 l-nrf/ida. 



The earliest stage thus far observed was found in a specimen 

 a little over four millimeters in length (plate x, fig. 1). The 

 loop at this time is composed of two broad descending lamella?, 

 which begin at the ends of the crura and extend forward, curv- 

 ing 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 Ccntronella, and this stage 

 is therefore called the cenlronclliform 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 

 reentrant in front (fig. 2). Further resorption in the same man- 

 ner results in the production of two posteriorly directed branches, 

 as shown in fig. 3. This form may be considered as an early 

 immature Dielasma loop, as subsequent growth does not mate- 

 rially modify its general characters. 



The adult loop, represented in figs. 4-6, differs from the 

 early Dielasma stage chiefly in the divergence of the descending 

 branches. 



In the centrouelliforni stage the lamella} converge, and the loop 

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

 gradually alter until, in the early Dielasma stage, the descend- 

 ing branches are nearly parallel, the loop extends less than half 

 the length, and, finally, when mature, the descending branches 

 diverge and the loop is two-fifths the length of the dorsal valve. 



The natural inferences to be drawn from the development of 

 the loop in Dielasma are, that Centronella represents a larval or 

 immature condition of the higher genera, and that the centronell- 

 oid loop is the primitive type in the Terebratulidse. There- 

 fore, as Centronella and the closely related genus Rensselaeria are 

 the only early punctate terebratuloids known, and as they have 

 the primitive type of loop, there arises the question of the val- 



