206 



MOLLUSCA. 



foramen. This genus extends from the Upper Silurian to the 

 Trias. In the genus Athyris (fig. 146) there is no hinge-area 

 or deltidium, and the beak of the ventral valve does not appear 

 to be perforated by a foramen in the adult condition. All the 

 species of Athyris appear to be Palaeozoic, and they are es- 

 pecially characteristic of the Devonian period. In the genus 

 Spirigera (including Retzia) there is no true hinge-area, and 

 the beak of the ventral valve is terminated by a round foramen, 

 with a distinct deltidium. The members of this genus extend 

 from the Silurian to the Trias. In the genus Uncites the shell 



Fig. 149. Atrypa(Spirigerina)reticularis, Silurian. 



is impunctate, there is no hinge-area, and the ventral valve has 

 a long incurved beak, which is perforated in the young shell 

 by a small foramen. The genus is, so far as known, exclusively 

 Devonian. Lastly, in the genus Atrypa (fig. 149) the shell is 

 impunctate, and the beak of the ventral valve is long and 



incurved. The beak is perforated 

 by a small foramen, completed by a 

 deltidium. Owing to the curvature 

 of the beak, however, this foramen is 

 often concealed, so that the beak 

 seems to be imperforate ; hence the 

 name of the genus (Gr. a, without ; 

 trupa, aperture.) It is probable that 

 the shell was unattached in the later 

 portions of its existence. The genus 

 is exclusively Palaeozoic, appearing 

 to be confined to the Silurian and 

 Devonian formations. The best- 

 known species is A. reticularis, figured above. 



FAM. IV. KONINCKINID^E. Animal unknown. Shell free ; 

 valves unarticulated (?). Oral arms supported by two lamellae 

 spirally coiled (fig. 150). 



The only genus of this family is Koninckia, represented by 



Fig. 150. J\omticKui Leon- 

 Jtardi, showing the spiral sup- 

 ports for the arms. Trias. 



