THECIDEA. 581 



Sub-gen. cistella, Gray. 



Shell with the loop two-lobed. 



cistellula, Searles Wood. Neapolitana, Scacchi. 



cuneata, Risso. 



Genus THECIDEA, Defiance. 



Shell thick, rounded or oval, more or less regular, punc- 

 tate, attached by the umbo of the ventral valve, or free ; 

 ventral valve with the umbo rather produced and perforated 

 by a minute apical foramen usually closed at an early age ; 

 hinge-area flat, triangular; deltidium triangular, scarcely 

 distinct from the area ; interior deeply concave, furnished 

 with two prominent cardinal teeth, the surface striated and 

 marked with three muscular scars ; dorsal valve small, 

 rounded, opercular, furnished interiorly with a prominent 

 cardinal process between the dental sockets. Oral processes 

 united, forming a bridge over the visceral cavity, separating 

 the oral from the anal orifice ; disk grooved for the recep- 

 tion of the loop, the grooves separated by branches from a 

 centra] septum ; loop often unsymmetrical, divided into two 

 or more lobes, united more or less intimately with the sides 

 of the grooves. 



Si/71. Thecidium, Sowerby. 



Ex. T. Mediterranea, Risso, pi. 132, fig. 1. Shell, T. 

 Mediterranea, fig. I, a, 1 b. 



In the genus Thecidea the shell is attached by the trun- 

 cated apex of the dorsal valve, or it is free, when the apex 

 is produced and entire. The calcareous loop is folded into 

 two or more lobes, which repose in hollows of corresponding 

 form excavated in the substance of the smaller valve. 



vol. it. . J i 



