38 SHELL GALLEKY. 
or attach themselves by a byssus, sometimes forming a sort of nest, 
consisting of pieces of coral and shell or small stones, in which they 
are completely concealed. 
Cases 156- The Scallops or Fan-shells (PecU/iidce) are well known for their 
beautiful colours, sculpture, and excellent flavour. The animal has 
a distinct foot, which is not, however, used as a locomotive organ, 
but employed in spinning a byssus of attachment when required. 
The young Pectens dart through the water by opening and suddenly 
closing their valves. The species are very numerous, world-wide in 
their distribution, and may be found at depths from a few to three 
thousand fathoms. 
EULAMELLIBRAXCHIA. (Fig. 25, C.) 
Cases 160- j n ^ s order the gills have intern* lamentar and interlamellar 
vascular junctions, and there are generally two adductor muscles. 
The order is very extensive, comprising nearly sixty families, of 
which only the more important can be referred to. 
The Car dit idee and Astartuke have strong solid shells, frequently 
ornamented with radiating or concentric ribbing, and usually are 
coated with a dark epidermis. They have the general appearance 
of certain Veneridce ; but the animal has no prolonged siphons, but 
merely a fringed opening in the mantle. One very remarkable 
species, Thecalia concamerata (Case 161), has an internal cup-like 
process within the valves, which serves as a nursing-pouch for the 
young. 
Cases 162- The Lucinidcc are almost invariably white shells, and may 
163, generally be recognized by the very long muscular scar in front 
on the inner surface of the valves. They occur in all parts of the 
world ; and the fossil forms, which are still more numerous than 
those now living, have existed at every epoch from the Silurian. 
Cases 166- Of the freshwater Mussels or Unionidce more than 1200 species 
have been already discovered ; they are found in most parts of the 
world, the greatest number having been described from North 
America. In Unio the edges of the mantle are not united along 
the bottom and not prolonged into siphonal tubes ; at the posterior 
end there are two openings, of which the upper or excretal orifice 
is simple, and the lower or branchial fringed at the edge. Two 
" cephalic eyes " have recently been noticed by Mr. P. Pelseneer. 
The foot is very large and adapted for crawling and burrowing. 
The sexes are distinct ; and the shells of the females are somewhat 
177 
