224 



MANXTAL OF THE MOLLUSCA. 



Types, A. concentrica, Bucli. A. Roissyi, fig. 133, 134. A. lameUosa, 

 PI. XV. fig. 16. 



Shell impunctate, transversely oval, or sub-orbicular, bi-convex, smooth, 

 or ornamented with squamose lines of growth, sometimes developed into 

 win^-like expansions, (fig. 134:*) ; hinge-line curved, area obsolete, foramen 



Fig. 133. Interior of dorsal valve. Fig. 134. Specimen with fringe. 



round, truncating the beak, deltidium obsolete ; hinge-plate of dorsal valve 

 with fom' muscular cavities, perforated by a small round foramen, and supporting 

 a small complicated loop (?) between the spires ; spires directed outwards, 

 crura united by a prominent oral loop. 



The foramen in the hinge-plate occupies the situation of the notch 

 through which the intestine passes in the recent RhpichonellcB ; in A. con- 

 centrica a slender curved tube is sometimes attached to the foramen, beneath 

 the hinge-plate. A. tmnida has the hinge-plate merely grooved, and the 

 byssal foramen is angular. 



Fossil, about 20 sp. Silurian — Lias. N. and S. America ; Europe. 



Sub-ffenus? Merista, Suess. Ter. scalprum, Roemer, 

 (A. cassidea, Quenst. Sp. plebeia. Ph.) Silurian — 

 Devonian ; Europe. Shell impunctate, dental plates 

 (f) and dorsal septum (d) supported by arched plates 

 (" shoe-lifter" processes, of King) which readily det- 

 ach, leaving cavities (as in fig. 135) ; spiral arms have 

 been observed in aU the species. 



Retzia, King. 



Dedicated to the distinguished Swedish naturalist, Retzius. 



Type, Ter. Adrieni, Vern. £x. R. serpentina, Carb. L. Belgium. Eig. 136 



Shell punctate, terebratula-shaped ; beak truncated by a round foramen 

 rendered complete by a distinct deltidium: hinge-area small, triangular, 

 sharply defined ; interior with diverging shelly spires. 



Fossil, about 20 species. Silurian — Trias. S. America. U. S. Europe- 



* The spurious genus Actinoconchus (M'Coy) was founded on this character; 

 similar expansions are formed by species of Atrj-pa, Camarophoria, and Products. 



Fig. 135. Merista. 



