MONOGRAPH OF THE GENUS 



GASTROCH.ENA, Lamk. 



Molluscum perforans, tubam formans. Valvoe modioli- 

 formes antice ad marginem ventralem late hiantes,ligamentum 

 externum ; umbonibus antice fere terminalibus. Tuba subtes- 

 tacea bulbiformis agglutinans, interdum libera. — The valves 

 of this burrowing and boring jNIollusc are wedge-shaped, 

 being short and narrow anteriorly, with the umboes almost 

 terminal. The tube generally forms a testaceous lining to the 

 hole bored by the animal ; when free it is shaped like a flask 

 or bottle, closed at the large end, and divided at the small 

 open end in correspondence with the double siphon. 



Species. 



1. uuBiA, f. 2, Pennant. — Elongata, tenuis, perobliqua, 

 tenuissime striata ; hiatu mediocri ; cardine fere edentato. 

 Tuba bulbiformis, agglutinans. 



2. DiFFiciLis, f. 4, Defihayes. — G. dubice similis, sed postice 

 subcarinata, ventricosa, lata, ad terminum acuminata. — Dif- 

 fering from sp. 1 in being obliquely keeled, rather pointed 

 posteriorly, and broad and swollen on the postero-ventral 

 margin. 



3. APERTissiMA, f. 5, DesJuiyes. — Elongata, angusta ; mar- 

 gine dorsali subelevato, ventrali post medium depresso ; 

 hiatu ventrali fere ad terminum extenso. — Remarkable for 

 the hiatus, which reaches nearly to the end of the sliell. 



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