THE BRACHIOPODA 15 



are no crura even, but the thick dorsal valve has deep 

 furrows with ridges between corresponding to the course 

 of the spiral brachia. The other structures do not differ 

 essentially from those of rhynchonellids and terebratuloids, 

 but evidently this is something distinct from' both. In 

 particular, the single deltidium in place of two deltidial 

 plates is a more important difference than might at first 

 sight appear, because the study of the early development 

 has shown that these structures appear as secretions by 

 different parts of the body the single plate, by the pedicle 

 itself; the double plates, by the ventral mantle-lobe. 

 Only two species of Thecidea are known to be living now, 

 but many fossil species are known, though on account of 

 their small size they are often overlooked. Among modern 

 forms Thecidea is an isolated genus with no near allies ; 

 but there are many extinct forms more nearly related to 

 it than to any other living genus. 



Discinisca lamellosa (Fig. 7, c) is found in shallow water 

 or between tide-marks on the coast of Peru. Its shell 

 differs both in texture and in shape from those hitherto 

 considered, for it is not calcareous but has a horny 

 appearance and consists of organic matter with some 

 calcium phosphate instead of carbonate ; and each valve 

 is nearly circular in outline, with the umbo near the 

 centre. The ventral valve resembles a low cone of 

 which one portion of the surface (on the posterior side) 

 has been crushed in. In the middle of this depressed 

 area is a narrow slit through which the pedicle passes 

 obliquely. The dorsal valve is a still flatter cone. 

 Internally, there are no hinge-teeth, no cardinal process 



