260 STUDIES IN EVOLUTION 



tral mantle lobe must have formed the shell in the usual 

 way. This appears all the more probable from the fact that 

 the lower or ventral valve is punctate, and, so far as known, 

 the mantle contains all the csecal prolongations, which alone 

 could produce the punctate structure. Careful microscopic 

 examination has failed to detect punctse in the deltidia of 

 Thecidium, Strophomena, Leptcena, and other punctate genera 

 belonging to the Protremata. 



It is true that Aulosteges has spines on the deltidium, but 

 spines even when tubular are not equivalent to punctse, 

 as shown in Producing Strophalosia, and some species of 

 Spirifer. Aulosteges is a gerontic genus, which has become 

 excessively spinose, and has also reverted to ancestral char- 

 acters in its high hinge-area and conspicuous deltidium. It 

 is well known that even the spires of Spiriferina and the loop 

 of Macandrevia are spinose. 



Turning now to Cistella as a representative of the Telo- 

 tremata, a different process obtains. 



Figure 108 represents the fully developed, free-swimming 

 cephalula of Cistella, and shows the extent of the folds of 

 the mantle and their posterior direction. Figure 109 repre- 

 sents the same in section. The inner sides of the mantle 

 lobes are to form the future valves, the dorsal (ds\ and the 

 ventral (vs) . The transformed larva or typembryo is repre- 

 sented in figure 110 and in section in figure 111. It is seen 

 that the transformation consists in the folding forward of 

 the mantle lobes over the head segment (A). Now the shell- 

 secreting layers of the mantle are exterior, and the two 

 valves begin to form, the dorsal shell (c?s), and the ventral 

 (vs). The pedicle and posterior portion of the body come out 

 freely between the valves and mantle lobes and limit the 

 hinge-areas of both (hi and M). 



The further process of growth increases the distance be- 

 tween the initial dorsal and ventral hinges, for while the 

 original dorsal beak is usually maintained at the hinge-line, 

 the ventral beak is progressively removed and the ventral 

 hinge travels from its first position at the beak, along the 



