Report of the State Geologist. 



539 



extremities; the ventral valve meets the deltidium and becomes 

 anchylosed with it, leaving an opening for the passage of the 

 pedicle. The union of the three shell-plates in the phylembryo 

 forms the completed embryonic shell or protegulum. 



In CistbllAj and presumably in all articulate brachiopods 

 possessing deltidial plates, the mode of development of the shell 



Fig. 217.— Dethyrium of 

 a young Rhynchonella 

 without deltidial plates. 



Fig. 218.— The same at 

 a later stage with two 

 triangular deltidial 

 plates. 



Fig. 219.— The same at 

 completed growth of the 

 deltidial plates. 



Figs. 220 and 221.— Dorsal and profile views of Magellania. Jlave*cen8 % showing 

 deltidial plates and pedicle. 



Fig. 222.— Dorsal view of the umbonal por- 

 tion of an adult Terebratulina septen- 

 trionalis with the shell removed by acid: 

 showing slight secondary extensions of 

 the central mantle around the pedicle, 

 small deltidial plates 'only, being secreted 

 in this species. 



Fig. 223.— Similar preparation of Magellania 

 flavescens; showing the complete envelop- 

 ment of the base of the pedicle by secondary 

 expansions of the ventral mantle which have 

 formed the deltidial plates, as shown in 

 fig. 220. (After Beeqher.) 



is quite different. The ventral lobe of the mantle is here the 

 longer, and no body-plate is formed on either side of the thoracic 

 lobe. When, therefore, the mantle lobes are reversed and the 

 valves come into contact about their peripheries and at the cardinal 

 extremities, the caudal lobe, or pedicle, occupies all the space 

 between the posterior margins of the valves. The large, 



91 



