BRACHIOPODA. 



205 



The members of this family are all attached to some foreign 

 body by a portion of the beak of the ventral valve, which, 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 present day 

 by a single living species. In time, the genus TJiecidium 

 seems to have commenced in the Upper Trias, and is well re- 

 presented in parts of the Jurassic and Cretaceous Series. 



FAM. III. SpiRiFERiDjE. Animal free when adult, or rarely 

 attached by a muscular peduncle. Shell punctated or un- 

 punctated. Arms greatly 

 developed, and entirely 

 supported upon a thin, 

 shelly, spirally-rolled la- 

 mella (fig. 142 and fig. 

 146.) 



The family of the Spir- 

 iferidtz is pre - eminently 

 Palaeozoic, but several 

 forms extend into the older 

 Secondary Rocks. No 



member of the family, however, has yet been found in rocks 

 younger than the Lias. Of the genera of the family, in the gen- 

 us Spirt/era, or Spirifer (fig. 148), the valves of the shell are 

 articulated by teeth and sockets, and the shell is not punctated. 

 The hinge-area is divided across in each valve by a triangular 

 fissure, which in the ventral valve is closed more or less com- 



Fig. 146. Atkyris sitbtilita. Lower Car- 

 boniferous. The right-hand figure shows the 

 interior of the dorsal valve, with the spiral sup- 

 ports for the arms. (After Dawson.) 



Figs. 147 and 148. Spirifera. sculptilis, Devonian. Sfarifera. mucrouata, Devonian. 



pletely by a pseudo-deltidium, and in the dorsal valve is 

 occupied by the cardinal process. The true Spirifers are 

 mainly Silurian and Devonian, and the forms of the latter 

 formation often have the shell winged, or drawn out at the 

 lateral angles (fig. 148). The genus Spiriferina differs from 

 Spirifer chiefly in having the shell punctated, and extends 

 from the Devonian to the Lias. In the genus Cyrtia, the 

 shell is impunctate, and the deltidium is perforated by a small 



