Fig. 328.—Terebratula sella. (Lower Greensand.) d, Deltidium; f, foramen.
The dorsal valve in a few cases has its beak perforated by a 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 Centronella 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 (Waldheimia, Fig. [329], ss; and Stringocephalus, Fig. [326], B, v.s). Conchidium (Fig. [325]) has its dental plates of great size, and uniting to form a V-shaped chamber or “spondylium,” supported by a median double septum; and by means of these with a pair of septa and the large socket-plates in the dorsal valve the interior of the shell of this genus is divided up into several chambers.
The interiors of several other genera are somewhat similarly divided up.
Fig. 329.—Waldheimia (Magellania) flavescens. A, Interior of ventral valve: a, adductor scars; v.a, ventral adjustors; d, divaricators; a.d, accessory divaricators; p, peduncular muscle; dm, deltidium; f, foramen; t, teeth. B, Interior of dorsal valve: a.a, anterior adductor (occlusor) scars; a.p, posterior adductor (occlusor) scars; c.p, cardinal process; cr, crura; d.s, dental sockets; hp, hinge-plate; l, brachial loop; ss, septum. (After Davidson.)
In the Carboniferous genus Syringothyris two special plates, situated between the dental plates, are rolled into an incomplete tube, so as to enclose probably the anal extremity of the alimentary canal; and in several genera a sub-umbonal “cardinal plate” is present, which is perforated (Athyris) or slit in some cases for the passage of the anal tube.
For the support of the fleshy “spiral arms” the calcareous structures forming the “brachial apparatus” are of two main types—(1) the loop type; (2) the spiral-cone type. In the Strophomenidae no special calcareous support seems to have been usually present (Fig. [334]), though in some species of Leptaena spirally-grooved elevated areas supported the fleshy arms; in the Productidae it is probable that the ridges enclosing the “reniform impressions” (Fig. [333], i) served for a similar purpose.