METAZOA VERMES. 281 



.the little box are somewhat flattened and the beak of the 

 pedicle valve is in a nearly normal condition. The pedicle 

 opening is large, and the delthyrium is not filled with the 

 deltaria. The older shells on the standard are thickened 

 and have a sharp anterior edge. The pedicle has worn 

 away a portion of the beak, while the deltaria have formed, 

 and the suture indicating their line of contact can be 

 plainly seen in the specimen in the lower right hand 

 corner. In old age the shell becomes so rotund that the 

 valves when separated are bowl-like in shape (see side 

 view of the shell in the upper right hand corner). Each 

 valve is thickened along the margin by many layers 

 crowded closely together. The suture of the deltaria 

 has disappeared, leaving apparently a single plate, the 

 deltarium or pseudodeltidium. The brachial loop or ring 

 is essentially the same in the youngest and oldest stages. 



It now seems probable that Tropidoleptus carinatus 

 Conr. (PI. 695 ; No. 696) is an ancestral form of the 

 family Terebratellidae. It is a thin shell with convex 

 pedicle and concave brachial valve. The cardinal area 

 is straight and narrow, and in the young (PI. 695, fig. i) 

 longer than the greatest diameter of the shell, but in the 

 adult (No. 696; PI. 695, figs. 2-4) it is shorter. The 

 pedicle passage is never closed by a deltidium (fig. 4 ; 

 fig. 5, side view of cardinal area enlarged), but a chili- 

 dium is well developed on the brachial valve (fig. 5). 

 The loop (fig. 6) consists of two descending branches or 

 lamellae united to the median septum. These parts are 

 still better seen in fig. 7, which is a side view of the sep- 

 tum loop and the two jugal processes extending from the 

 lamellae. 



Cistella neapolitana Scacchi, belonging to the generalized 

 Terebratellidae, is living to-day. Its development tends 

 to prove that Brachiopods are closely related to Worms. 

 No. 697, with models 1-10, and pi. 698, figs, i-io, illus- 

 trate the development of this genus. The egg (No. 697, 

 i) is unsegmented. Its division into two spheres is seen 



