54 EOCENE MOLLUSCA. 



from Barton, in which it is so, the right or flatter valve exhibits faint radiations ; 

 in one specimen of Mr. Edwards, (fig. 1, c,) of the left valve, these radiations are not 

 visible, although the outer layer is well preserved and perfect, showing irregular and 

 lamellated and somewhat undulating lines of growth. 



3. Avicula papyracea, /. Sowerb//. Tab. XI, fig. 2, a — c. 



Avicula papyracea. J. Sow. Geol. Tr., 2d ser., vol. v, pi. 8, fig. 1C, 1834. 

 — — Morris. Cat. Brit. Foss., p. 104, 1854. 



Spec. Char. Jv. testa tenuissimd, papyracea, obliqitd suborbiculari, compressd, con- 

 centrice costulatd aut undulatd ; auriculis minimis, inaqua/ibus. 



Shell very thin, papyraceous, obliquely orbicular ; much compressed ; concentrically 

 ridged or waved ; ears small, unequal. 



Longest diameter, \\ inch. 



Localities, b, Hampstead, Sheppy, Primrose Hill {Wether ell), f. a. Haverstock 

 Hill {Edwards). 



"An extremely thin, pearly shell, sometimes assembled in considerable masses." 

 {Sowerby.) Although specimens are by no means rare, the shell is always compressed and 

 distorted ; indeed, all I have seen are literally flat. In outward form it is unlike the 

 generality of shells of this genus, in having the hinge-line but little developed ; it resembles 

 a Posidonia in that character. A specimen in the Museum of the Geological Society, ap- 

 parently belonging to this species, is marked Southend {Warburton). 



PINNA. Linnams, 1767. 



Generic Character. Shell equivalve, cuneiform, or wedge-shaped ; umbones at the pedal 

 extremity, siphonal region truncated and gaping; connexus ligamentous, linear ; hinge 

 edentulous ; adductor muscle-mark subcentral, large, ill defined. 



Animal the shape of the shell ; the margins of the mantle with a double fringe, and 

 disunited except at the dorsal edge ; foot elongated, grooved, spins a large, powerful, and 

 silky byssus ; attached by triple muscles to the centre of each valve. 



This is a marine genus. The species range from low-water mark to sixty fathoms, 

 and the animals live generally buried in the sand or mud, with the pointed end of their 

 valves inserted ; they are gregarious, large numbers congregating in one spot. 



The composition of the shell in this genus appears to be different from that in most of 

 the Mollusca, and, according to Dr. Carpenter, is made up of two very differently constituted 

 portions. The outer or larger part of the shell is cellular, or composed of fibrous material, 



