444 



MOLLUSCA. 



in the adult state, has either no foramen, or an exceedingly 

 small one. The ventral valve has a well-marked hinge-area 

 and an indistinct triangular deltidium. All the known species 

 belong to the single genus Thecidium, represented at the 



Lt 



Fig. 283. Thecidium papillatum. e, Hinge-area; n. Hinge-teeth of ventral valve ; 

 r, r, Granulated border of the interior of the dorsal valve. 



present day by a single living species. In time, the genus 

 Thecidium seems to have commenced in the Upper Trias, 

 and is well represented in parts of the Jurassic and Cretace- 

 ous Series. 



FAM. III. SPIRIFERID.E. 

 Animal free when adult, or 

 rarely attached by a muscu- 

 lar peduncle. Shell punc- 

 tated or unpunctated. Arms 

 greatly developed, and en- 

 tirely supported upon a thin, 

 shelly, spirally-rolled lamella 

 (figs. 279 and 284). 



The family of the Spiriferidce, is pre-eminently Paleozoic, 

 but several forms extend into the older Secondary rocks. 

 No member of the family, however, has yet been found in 



Pig. 284. Athyris subtilita Lower Car- 

 boniferous. The right-hand figure shows the 

 interior of the dorsal valve, with the spiral 

 supports for the arms. (After Dawson.) 



