PLATE XX. 



Fig 



10. Terebratula Australis, }Q,uoy et Gaim. Perforate valve, twice the natural size. 



11- — — Imperforate valve of the same. 



12. — — Diagram of a longitudinal section (left half of the shell), 



representing the muscular system, &c., of the same. 

 (The various structures in the three figures have the same 

 reference letters.) 



A, Teeth or condyles. 



B, Dental sockets. 

 C, Socket plates. 



D, Crural base. 



jB'j Crura of the loop. 



F, Loop. 



G, Dorsal median plate. 

 H, Foramen. 



/, Deltidium (divided). 



/, Cardinal muscular fulcrum. 



a. Pedicle. 



b. .Superior pedicle muscles. 



c. Cardinal muscles. 



d. Valvular muscles. 



e. Inferior pedicle muscles. 



f. Impressions of the posterior divisions of the 



valvular muscles, 

 y. Do. of the anterior divisions. 



