224 



MANUAL OF THE MOLLUSCA. 



Types, A. concentrica, Buch. A. Roissyi, fig. 133, 134. A. lamellosa, 

 PI. XV. fig. 16. 



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

 or ornamented with squamose lines of growth, sometimes developed into 

 wing-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 four 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 jiinge-plate occupies the situation of the notch 

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

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

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

 byssal foramen is angular. 



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



Sub-genus? Merista, Suess. Ter. scalprum, Rcemer^ 

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

 Devonian; Europe. Shell impunctate, dental plates 

 (v) 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 all the species. 



RETZIA, King. 



Dedicated to the distinguished Swedish naturalist, Retzius. 



Type,Ter. Adrieni, Vern. Ex. R. serpentina, Carb. L. Belgium. Fig. 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 Atrypa, Camarophoria, and Producta. 



Fig. 135. Merista. 



