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 Beecher. These plates lie on 

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

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

 (^Terebratula, 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. — Rhynchonella 

 Boueti. (Cornbrash.) 

 d, Deltidium; /, fora- 

 men. 



Fig. 328. — Terehratula sella. 

 (Lower Greensand.) d, Del- 

 tidium; /, foramen. 



foramen — the "visceral foramen." This foramen is in no way 

 connected with the pedicle foramen, but points perhaps 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 Oentronella the 

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

 Strophomena 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- 



