374 Mr. T. Davidson on the Classification 



X. Genus Thecidea, Def. 1828. 



Thecidea has a calcareous loop folded into two or more lobes, 

 and lying in hollows of correspond- 

 ing form excavated in the substance 

 of the small valve. This loop, or 

 apophysary ridge, supports the 

 branchial membrane, whose thick- 

 ened and ciliated margin is appa- 

 rently attached to the inner sides 

 of the grooves. The cilia are very 

 long, especially the outer series, 

 which are directed inwards in the 

 dried specimens. 



47. Thecidea mediterranean Risso^ 1826 ; Hist. Nat. des Princ. 

 Prod, de PEurope Mer. fig. 183 ; Sow. Th. Conch, pi. 73. 

 fig. 30-32. 



Hab. Mediterranean. 



2. Family Sptrifertd^e (extinct). 



Shell furnished with spiral calcareous processes for the sup- 

 port of the oral arms. Usually trilobed and winged. 



3. Family Rhynchonellid^. 



Shell not punctate, usually tetrahedral and sharply plaited ; 

 oral arms spiral, supported only by short curved processes. 



XI. Genus Rhynchonella, Fischer, 1809. 



Shell variable in shape ; beak acute, more or less recurved, no 

 true area ; foramen variable in its dimensions and form, placed 

 under the beak, exposed or concealed, entirely or partially sur- 

 rounded by a deltidium in two pieces ; apophysary system in 

 smaller valve composed of two short flattened and grooved 

 lamellse separate and moderately curved upward, attached to the 

 inner side of the beak of smaller valve, and to which were afiixed 

 the free fleshy arms. 



Ohs. Not having had the advantage of examining the animal of 

 this genus, I can only refer to Prof. Owen^s anatomy of R.psit- 

 tacea in the Zool. Trans, vol. i. 2nd part ; two species of recent 

 Rhynchonella are known. 



48. Rhynchonella psittacea, Chem. sp., 1785; Sow. Th. Conch, 

 pi. 71. fig. 78, 79, 80. 



Hab, Labrador, in the mud on the shore at low water : dredged 

 alive at Melville Island, throughout the arctic seas, from low 



