500 



FOSSIL BRACHIOPODA 



CHAP. 



heimia^ Fig. 329, ss; and String ocephaliis^ Fig. 326, B, v.s). Oon- 

 ehidium (Fig. 325) has its dental plates of great size, and unit- 

 ing 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. 



In the Carboniferous genus Syringotliyris two special plates, 

 situated between the dental plates, are rolled into an incomplete 

 tube, so as to enclose probably the anal extremity of the ali- 

 mentary canal ; and in several genera a sub-umbonal " cardinal 



Fig. 320. — Waldheimia {Magellania) flaveacens. A, Interior of ventral valve: «, 

 adductor scars ; v.a, ventral adjusters; d, divaricators ; a.d, accessory divaricators ; 

 p, peduncular muscle; dm, deltidium ; /, 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; I, 

 brachial loop; ss, septum. (After Davidson.) 



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 

 Stropliomenidae 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 " reni- 

 form impressions " (Fig. 333, i) served for a similar purpose. 



The Terehratulidae show the " loop type " of brachial appa- 

 ratus. In Waldheimia (Fig. 329), which may be taken as an 



