120 



a modification of the kind of laminar beard formed by the end of the 

 foot of the Arcs (Arcce) ; for, hke it, it is formed of numerous parallel, 

 erect, longitudinal horny laminae, placed side by side, extending from 

 the apex to the margin, and it is on these plates that the calcareous 

 matter is deposited when the attachment assumes its shelly substance. 

 The same structure is to be observed in the plug of the European 

 Anomia Ephippium {striata)." — Voy. New Zealand, ii. 261. 



Messrs. Forbes and Hanley compare it to the byssus of Pecten, 

 and venture to predict that when the very yomig Anomise have been 

 observed, they will be found to be attached by threads like that genus 

 {Brit. Moll.). I have examined a very small specimen of the genus, 

 and found it laminar, like that of the adult shell. 



M. Phihppi, when describing Pododesmus, appears to have ob- 

 served only the upper of the two muscular scars, for he gives as the 

 generic character, " Impressio imiscularis unica, ovata," and he only 

 figures the larger upper one on the plate. 



The upper scar, which is usually of a larger size, and has its surface 

 covered with radiating veins, while the lower is generally punctated, 

 appears to be the one which gives rise to the muscle that is attached 

 to the inner surface of the plug. 



* Shell plicately folded. Perforation of lower valve small, firmly 

 embracing the plug. Placunanomia. 



1. Placunanomia Cumingii. 



Shell depressed ; edge of the valves with three or four large angular 

 folds. 



Placunanomia Cumingii, Broderip, Proc. Zool. Soc. 1832, 29 ; 

 Sow. Genera, t. ; Manual, t. . f. 



Hab. Central America ; Gulf of Dulce, Province of Costa Rico. 



** Shell ovate, radiately ribbed; edge not plicated. Perforation of 

 lower valve moderate, firmly embracing and inclosing the plug. 

 Pododesmus. 



i" American. 



f3 A/ 2. Placunanomia rudis. 



White ; disk brown ; smooth laminae. 



Upper valve with two rounded separate scars of nearly equal size, 

 the liinder one rather more transverse. 



Placunanomia rudis, Broderip, Proc. Zool. Soc. 1834, 2. 



Pododesmus decipiens, Philippi, Wiegmann Arch. i. 1837, 387. 

 t. 9. f. 1 (one scar left out). 



Hab. East Indies ? Broderip. Havana ; Philippi. West Indies ; 

 Brit. Mus. 



(J^f\ 3. Placunanomia foliata. 



White, smooth laminae, with very slight, distant, radiated ribs ; 

 disk purple brown. 



Upper valve with two nearly united scars ; the upper largest, and 

 rather elongated ; lower small, rounded. 



