500 



FOSSIL BRACHIOrODA 



heimia, Eig. 329, ss ; and Stringocephalus, Fig. 326, B, us). Con- 

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

 ing to form a Y-shaped chamber or " spondylium," supported by a 

 median douljle septum ; and by means of these with a pair of septa 

 and the large socket-plates in the dorsal valve the interior of 

 the slicll of tliis genus is divided up into several chambers. 



Tlie interiors of several other genera are somewhat similarh' 

 divided up. 



In the Carboniferous genus Syringothyris two special plates, 

 situated between the dental plates, are rolled into an incomplete 

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

 mentary canal ; and in several genera a sub-uiuli(iii;d " cardiu:d 



Fig. 329. — Waldheimia {MageUania)Jlavescens. A, Interior of ventral valve : a, adductor 

 scars ; v.a, ventral adjustors ; d, divaricators ; a.d, accessory divaricators ; jJ, pedun- 

 cular muscle ; dm, deltidiuin ; /, foramen ; t, teeth. B, interior of dorsal valve : 

 a.a, anterior adductor (occlusor) scars ; a.p, posterior adductor (occlusor) scars ; 

 c.^j), cai'dinal process ; cr, crura ; d.s, dental sockets ; hp, liinge-plato ; I, Lracliial 

 looii ; ss, septum. (After Davidson. ) 



plate " is present, whicli is perforated (AtJtyris) or slit in some 

 cases for the passage of the anal tube. 



For the support of the flesliy " spiral aruis " the calcareous 

 structures fornung the " l)rachial apparatus " are of two main 

 types — (1) the loop type; (2) the spiral-cone type. In the 

 Strophomenidae no special calcareous support seems to liaA'e 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 Terehratididae show the "loop type" of bracliial appM- 

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



