The Ark Shells. Chest Shells 



its hump just fits the hand of the small architect. Height, 2 

 inches. Length, 2^ to 3 inches. Rhode Island to Georgia. 



The Ponderous Ark (A. ponderosa, Say) is exceptionally 

 short and thick, in body and shell. The end view is symmetric- 

 ally heart-shaped. The strong ribs are crossed on the marginal 

 half by a few concentric lines, deeply impressed. The lips meet 

 in a straight line. The beaks are distant, prominent and inclined 

 to turn forward. The margins bear a wide border of dark, furry 

 epidermis. Length, 2\ inches. Fossil valves of this shell are 

 picked up on New Jersey beaches. 



Habitat. — Cape Cod to Texas, West Indies. 



Noah's Ark {A. Noce, Linn.) belongs to the group of ark 

 shells which attach themselves by a byssus to rocks. For this rea- 

 son they were called by Swainson, Byssoarca, to distinguish them 

 from the free-swimming species. These byssoarks throw out a 

 glandular secretion, comparable to the horse mussel's tough rope, 

 and the silken cables of Pinna. The byssus of the ark, however, 

 hardens into a horny cone, made of thin plates. This the ark can 

 cast off with great suddenness, and as promptly secrete 

 another. 



A Noah's ark is at best an irregular box. The prominent 

 umbones are separated by a wide dorsal depression above the 

 straight hinge line. They are near the anterior end of the shell, 

 which slopes downward like the prow of a dug-out. A widen- 

 ing groove extends from the beak of each valve to the abruptly 

 truncated rear margin. The hinge has about fifty fine teeth. The 

 ventral margin gapes midway to let the byssus out. Farther back 

 the white mantle edges are dotted with ocelli — eye-spots — small 

 brown elevations, each made up of many facets. On one speci- 

 men of Noah's ark Patten counted 235 of these compound eyes, 

 of varying sizes, in the two mantle borders. With these well-devel- 

 oped sight organs may be mentioned the presence of red blood. 



The mollusk scrambles about in rock crevices, grubbing in 

 crannies and among rubbish, and in its tender years the shell 

 often gets "warped for life." The ornamentation, too, gets badly 

 worn. Most specimens cast ashore are smooth, especially about 

 the beaks. A perfect shell is covered with strong ribs, marked 

 with tigerish streaks of brown and yellow. Inside, the colour is 

 plain lavender. Length, 3 to 5 inches. 



Habitat. — Mediterranean Sea, North Carolina to West Indies. 



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