49 



out for the purpose of anchorage, through a perforation in the lower valve, 

 but the orifice, instead of being open to the hinge, is soldered across the 

 top by a rude triangular ridge, or pair of converging teeth, of which a more 

 symmetrical development appears in the hinge of P lacuna. In the curious 

 species from central America, Placunanomia Cumingii, upon which the 

 genus is founded, the orifice is very contracted, and the calcifying muscle 

 insinuates itself between the laminse of the shell depositing a plate there ; 

 but this is only a peculiarity of the species. Taking the soldering tri- 

 angular ridge as the distinguishing characteristic of the genus, a series of 

 fourteen species become associated together, all of which may be recog- 

 nized at a glance from the true Anomia. 



The greater part of the Placunanomia are from the Central American 

 continent and the West Indies, but there are some fine species from Aus- 

 tralia and New Zealand, and the old Linnsean Anomia patelliformis of our 

 own shores comes into the group. 



1. abnormalis, Gray. 



2. alope, id. 



3. Australica, Reeve. 



4. cepio, Gray. 



5. Cumingii, Brod. 



Species. 



6. echinata, Brod. 



7. foliata, id. 



8. Gouldi, Reeve. 



9. Harfordi, id. 

 10. ione, Gray. 



11. macrochisms, Desk. 



12. patelliformis, Linn. 



13. rudis, Brod. 



14. Zealandica, Gray. 



Figure. 



Placunanomia ione. PI. 26. Fig. 157. Shell, turned over to show 

 that the orifice compared with that of Anomia, in the adjoining figure, 

 is soldered across the top. The orifice is partially filled up by the 

 plug of adhesion. The converging hinge-teeth being internal, are not 

 seen in the figure, but some idea of them may be gathered from the 

 tooth of Placuna shown in Pig. 158. 



Genus 3. PLACUNA, Bruguiere. 



Animal ; unknown. 



Shell; nearly equivalve, thin, semitransparent but firm, compara- 

 tively smooth; hinge consisting of two slender ridges in the 

 lower valve, converging at the ligamentary margin. 



In this genus are comprised the few large semitransparent shells com- 



VOL. II. 



