MOLLUSCA. 91 



differ greatly from one another and from the generic char- 

 acter. Some are found recent on our shores, while others 

 occur only in a fossil state. Lamark, having rectified the 

 Linnaean character of the genus, has separated many species, 

 now grouped, into distinct genera. In the restricted genus 

 ANOMIA, the under valve has a hole or groove near the 

 beak, which is closed by a testaceous operculum. This ap- 

 pendage is fixed to rocks or stones, and has a ligament at- 

 tached to it. 



In the genus CRANIA, represented by the Anomia crano- 

 laris of Linnaeus, the under valve is pierced by three holes, 

 which are oblique and unequal. The genus GRYPHJEA was 

 constituted from the Anomia Gryphus of Linnaeus. The 

 inferior valve is concave, terminating in a spirally involuted 

 beak, projecting upwards ; the upper valve is small, and re- 

 sembles a lid. A transversely striated pit at the hinge con- 

 tains the ligament. The only recent species known is called 

 G, angulata. Many species are found in a fossil state in 

 the rocks of this country. 



Among the Anomia, Linnaeus placed the shells which 

 compose the genus TEREBRATULA, the characters of which 

 are so obvious and distinct. In this genus, which is inequi- 

 valve and regular, the beak of the larger valve is produced, 

 and pierced with a hole, through which the ligament of ad- 

 hesion passes. From the great extent of this genus in fossil 

 species, several subdivisions have been proposed. 



From the Anomia placenta of Linnaeus, Lamark has form- 

 ed his genus PLACUNA. The hinge is remarkable for two 

 teeth on the one valve, placed like the letter V, the base 

 toward the beak, and two impressions on the other valve. 

 It occurs in the Indian Seas. The natives polish it for or- 

 naments. 



