0. E. Beecher — Development of the Brachiopoda. 145 



wards becomes attached to the ventral valve, and is then con- 

 sidered as belonging to it. 



The further growth of the deltidium around the body and 

 pedicle, and its consequent extension into the cavity of the 

 ventral umbo, may explain the origin of the spondylium. 



Kovalevski 16 believed the ventral valve in Thecidium was 

 secreted by the expanded edges of the pedicle and the body 

 walls, whether or not this is so does not affect the interpreta- 

 tion of the origin of the deltidium. From the observations of 

 Lacaze-Duthiers, 16 it seems, however, as though the ventral 

 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 examina- 

 tion has failed to detect punctse in the deltidia of Thecidium, 

 Strophomena, Leptsena, 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 punctae, as 

 shown in Productus, Stropbalosia, and some species of Spirifer. 

 Aulosteges is a geratalogous genus, which has become exces- 

 sively spinose, and has also reverted to ancestral characters 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 Telotre- 

 mata, a different process obtains. 



Figure 24 represents the fully developed, free swimming 

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

 mantle and their posterior direction. Figure 25 represents 

 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 represented in figure 

 26 and in section in figure 27. It is seen that the transforma- 

 tion consists in the folding forwards of the mantle lobes over 

 the head segment h. Now the shell-secreting layers of the 

 mantle are exterior, and the two valves begin to form, the 

 dorsal shell ds, 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 anp hi' . 



The further process of growth increases the distance between 

 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 edges of the 

 umbo leaving an open triangular area or delthyrium in the 



