CHAPTER XXVI: THE ARK SHELLS. CHEST 



SHELLS 



Family Arcid.^ 



Shell heavy, regular, box-like, with strong epidermis: lig- 

 ament external; hinge with a series of comb-like teeth; mantle 

 open; foot large, bent, and deeply grooved; gills oblique, united 

 posteriorly to a web; ocelli in mantle margin; siphons wanting. 

 Warm seas. 



Genus ARCA, Linn. 



Shells equivalve or nearly so, oval, or rather four-sided, 

 strongly ribbed or cancellated, ventricose, covered with heavy 

 epidermis; hinge straight, with many small, transverse teeth; 

 beaks nearer anterior end of shell, separated by a wide, flat loz- 

 enge-shaped ligamental area; foot pointed, with a heel; mantle 

 supplied with marginal ocelli; gills long, narrow; hearts two; 

 blood red in some species; byssal gland wanting or well developed. 

 About one hundred and fifty species distributed in all warm 

 seas, from tide mark to 250 fathoms. 



The Bloody Clam {A. pexata, Say) is well known be- 

 cause of its abundance and size, and the fact that it has red blood, 

 a rarity among mollusks. The shell is solid, obliquely oval, with 

 thirty or more ribs radiating from the hinge over the knob-like 

 beak. The deep grooves are delicately cancellated. The epi- 

 dermis is thick, dark and bristly. Inside the margin of each 

 valve is a border of alternate ribs and grooves. The hinge has 

 a comb-like series of teeth. 



The bloody clam has no place among "economic mollusks," 

 I believe. But the shells are important items in the equipment 

 of the littlest children who spend happy hours building hills and 

 valleys on the white beach sand. The concave of that strong 

 scalloped shell makes of it a capacious scoop, and the curve of 



384 



^ 



