XVIII TESTICARDINES : INTERNAL CHARACTERS 50I 



example, we notice first in the dorsal valve the "crura" (cr), 

 from which arise the two " descending branches " which run 

 forwards and then are bent back to form the "ascending 

 branches " which are united by the " transverse band." In some 

 genera the " ascending branches " may be reduced to mere 

 points, and the " transverse band " become a median vertical 

 plate ; the " crura," too, may be fused so as to form a " crural 

 band " ; and the " descending branches " may be connected by a 

 cross band — the " jugal band." In String oceplialus (Fig. 326, Z, 

 s.p) the loop is furnished on its inner edge with radiating pro- 

 cesses ; and in Argiope the loop is simple, not reflected, and 

 fused with marginal septa ; while in the Thecidiidae it is more 

 or less fused with the shell itself, and with the mass of calca- 

 reous spicules secreted by the mantle. 



The "spiral-cone type " of brachial apparatus is found in the 

 Spiriferidae, Atrgpidae, and Koninckinidae^ and consists of two 

 spirally-enrolled calcified lamellae, forming two cones with their 

 apices directed laterally (^Spirifera^ Fig. 330), or towards the 

 interior of the dorsal valve QAtrypa^ Fig. 332), or towards 

 each other ( Glassid) ; or forming two flat spirals in the same 

 plane (^Koninckinidae), A "jugal band" is generally present, 

 but varies much in posi- 

 tion, and in some genera 

 has complicated posterior 

 processes. 



The RhyncJionellidae 

 have no loop or spiral 

 cones, but merely a pair 

 of short " crura." 



The principal modifica- 



. , , , Fig. 330. — Spirifera striata. (Carboniferous 



tions in the attachments Limestone.) Showing brachial spires. 



of the muscles in the Tes- 



ticar dines are illustrated by Produetus giganteus (Fig. 333), 

 Leptaena rhomhoidalis (Fig. 334), and Waldheimia Jlavescens 

 (Fig. 329). 



In Produetus (Fig. 333) we see in the ventral valve a pair 

 of dendritic occlusor, often called adductor, impressions and a 

 pair of large flabellate divaricator impressions. In the dorsal 

 valve the large " cardinal process " served for the attachment 

 of the divaricator, and a low median septum separated the den- 



