Bracldal Supports in Dielasma cotd .Zygosprira. 73 



of l)otli loop and s[)ire-l)earing stdpks (Ancvlol)rachia and Tleli- 

 co})eginata) 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. 



■ Derelupincnf of the Loop in Didd-sma tanjidn. 



Tbe earliest stage thus far observed was found in a specimen 

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

 1()()}> at this time is composed of two broad descending -lamelhc, 

 which l)egin at the ends of the crura and extend forward, curv- 

 ing ventrally vintil they unite in the niedian 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 rcntronellifonn stage. 



The first change in the form of the loop is brought about l)y 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 tlie production of two posteriorly directed branches, 

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

 immature Dielusiiia loop, as su])se(pient growth does not mate- 

 riall}^ modify its general characters. 



The adult loop, represented in figs. 4-0, differs from the 

 early Dielasma stage chiefly in tlie divergence of the descending- 

 branches. * 



In the ecntmiu'tllfonii stage the lamella' converge, and tlie loop 

 extends half the lengtli of the sbell. J^otli of these relations 

 gradually alter until, in the early Did/wna stage, the descend- 

 ing branches are nearly parallel, the looj) 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 loo^i in Didasnut are, that Centronella represents a larval or 

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

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

 fore, as Centronella and the closely related genus Rensselwria are 

 the only early punctate terebratuloids known, and as they have 

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



