TESTICARDINES : INTERNAL CHARACTERS 



499 



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

 Iteeoming 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 "deltidium" 

 — which are formed on lateral extensions of the ventral mantle 

 lobe, according to Beecher. These plates lie on each side of the 

 pedicle, or grow round and unite in front of it {Illtynchunella, 

 Fig. ."5 27), or constitute merely its anterior border (^Terebratxda, 

 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. •i-27.—PJiijnchnyieUa Boueti. 

 (Coriibrash.) </, Ueltiiliuin ; 

 / foramen. 



Fig. 328. — Terehratula sella. 

 (Lower Greeusand.) d, 

 Deltidium ; /, fonuuen. 



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 aper- 

 ture. In Athyris conccntrica (Devonian) this foramen is connected 

 internally with a cyliiuh-ical tube, which extends longitudinally 

 to about one-third the lengtli of the valve. In Ceiiironella the 

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

 Stroj^homena and its allies tlic opening lies between the cardinal 

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

 retrogression of the group since I'alaeozoic times not only in 

 numbers, l)ut in structure ; and other evidence points the same 

 wav. 



Internal Characters 



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

 by septa. A median septum occui's in one; or both valves of 

 many genera as a low ridge or strongly deveIo]ied partition ( Wall- 



