412 MANUAL OF THE MOLLt'SCA. 



The Rev. D. Landsborough observed the fry of P. opercuJar'Sf 

 wben less than the 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 through the water. One jerk carried them some yards, 

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

 a different tack." 



Ths shell of Peden 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. nohilis 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, PI. 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. Porbesianus, PL XVI., Fig. 9. 

 Shell hyaline, posterior ears obsolete, anterior prominent; 

 right valve flat, byssal sinus deep ; structure permeated by 

 microscopic tubuli, as in Lima. 



Amusium, Klein. Shell nearly equivalved, gaping in front 

 and behind ; smooth outside, generally marked with radiating 

 grooves inside. 



Distribution, 176 species. "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, PI. XVL, Fig. 11. (Ostrea lima, L.) 

 Synonyms, Plagiostoma (Lliiwyd), Sby. P. cardiiforme, 

 PI. XVL, Fig. 12. 



Shell equivalve, compressed, obliquely oval ; anterior sice 



