412 MANUAL OF THE MOLLUSUA. 
The Rev. D. Landsborough observed the fry of P. opercularis, 
when less than the size of asixpence, swimming in a pool of 
sea-water left by the ebbmg 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.” 
The shell of Pecten and the succeeding genera consists almost 
exclusively of membranous lamine, coarsely or finely corru- 
gated. Itis composed of two very distinct layers, differing in 
colour (and also in texture and destructibility), but haying 
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, Vola, Klem. P. quinque- 
costatus and other fossil species with concayo-convyex 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., Fig.8. Hinge obscurely 
toothed. 
Hinnites (Cortesii) Defr. P. pusio, Pl. XVI., Fig. 10. Shell 
regular and byssiferous when young; afterwards cementing its 
lower valye and becoming more or less irregular. 
Distribution, 2 species. 
Fossil, Trias? Upper Greensand —, Hurope. 
Hemipecten, A. Adams. H. Forbesianus, 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. 
Euample, Li. squamosa, Pl. XVI., Fig. 11. (Ostrea lima, L.} 
Synonyms, Plagiostoma (Llnwyd), Sby. P. cardiforme, 
PLY Le Bie al2. 
Shell equiyalye, compressed, obliquely oval; anterior side 
