XVIII 



TESTICARDINES : INTERNAL CHARACTERS 



499 



development as a shell-growth on the dorsal side of the animal, 

 becoming attached to the ventral valve subsequently. The 

 pedicle in many genera passes out through a special foramen in 

 the beak of the ventral valve ; and its proximal portion is often 

 embraced by a pair of small plates — the deltidial plates or " del- 

 tidium " — which are formed on lateral extensions of the ven- 

 tral mantle lobe, according to Beeclier. These plates lie on 

 each side of the pedicle, or grow round and unite in front of it 

 (^Rhyiicho7iella, Fig. 327), or constitute merely its anterior border 

 QTerehratula^ Fig. 328). In some cases this foramen becomes 

 closed in old age. 



The dorsal valve in a few cases has its beak perforated by a 



Fig. 327.— RhynchonpJla 

 Boueti. (Corubrash.) 

 d, Deltidium; /, fora- 

 men. 



Fig. 328. — Terehrntula sella. 

 (Lower Greetisaiid.) d, Del- 

 tidium ; /, foramen. 



fora,men — the " visceral foramen." This foramen is in no way 

 connected with the pedicle foramen, but points perha[)s to the 

 existence in the early Testicardinate genera of an anal aperture. 

 In Athyris concentrica (Devonian) this foramen is connected 

 internally with a cylindrical tube, which extends longitudinally 

 to about one-third the length of the valve. In Centroiiella the 

 aperture in the cardinal plate is rounded and complete; and in 

 StropJiomena and its allies the opening lies between the cardinal 

 processes. If this feature is correctly interpreted, it suggests a 

 retrogression of the group since Palaeozoic times not only in 

 numbers, but in structure ; and other evidence points the same 

 way. 



Internal Characters 



The interior of the shell is sometimes more or less divided up 

 by septa. A median septum occurs in one or both valves of 

 many genera as a low ridge or strongly developed partition QVald- 



