412 MANUAL OP THE MOLIUSCA. 



The Rev. T>. Landsborougli observed the fry of P. opercAdar s, 

 wlien less than tlie size of a sixpence, swimming in a pool of 

 sea-water left by the ebbing of the tide. " Their motion was 

 rapid and zig-zag ; they seemed, by the sudden opening and 

 closing of their valves, to have the power of darting like an 

 arrow throngh the water. One jerk carried them some yards, 

 and then by another sudden jerk they were oflF in a moment on 

 a different tack." 



The shell of Ptden and the succeeding genera consists almost 

 exclusively of membranous laminae', coarsely or finely corru- 

 gated. It is composed of two very distinct layers, differing in 

 colour (and also in texture and destructibility), but having 

 essentially the same structure, Traces of cellularity are some- 

 times discoverable on the external surface ; P. nobilis has a 

 distinct prismatic-cellular layer externally. (Carpenter.) 



Sub-genera, Neithea, Drouet, Yola, Klein. P. quinque- 

 costatus and other fossil species with concavo-convex valves 

 and distinct hinge-teeth ; the inner layers of these shells are 

 wanting in all specimens from the English chalk. 



Pallium, Schum. P. plica, PL XVI., Pig. 8. Hinge obscurely 

 toothed. 



Hinnites (Cortesii) Defr. P. pusio, PI. XVI., Fig. 10. Shell 

 regular and byssiferous when young ; afterwards cementing its 

 lower valve and becoming more or less irregular. 



Distrihution, 2 species. 



Fossil, Trias ? Upper Greensand — , Europe. 



Hemi/peden, A. Adams. H. Eorbesianus, PL XVI., Pig. 9. 

 Shell hyaline, posterior ears obsolete, anterior prominent; 

 right valve flat, byssal sinus deep ; structure permeated by 

 microscopic tubuli, as in Lima. 



Amusium, Elein. Shell nearly equivalved, gaping in front 

 and behind ; smooth outside, generally marked with radiating 

 grooves inside. 



Pisfribution, 176 sjiecies. World-wide; Nova-Zembla — Cape 

 Horn ; — 200 fathoms. 



Fossil, 450 species (including Aviculo-pecten). Carb.- 

 World-wide. 



Lima, Bruguiere. 



Etymology, lima, a file. 



Example, L. squamosa, PL XVI., Pig. 11. (Ostrea lima, Li 

 Synonyms, Plagiostoma (Llhwyd), Sby. P. cardiiform^, 

 PLXVL, Pig. 12. 



Shell equivalve, compressed, obliquely oval ; anterior si^e 



