EAST COAST MARINE SHELLS 



27 



violently opened. 



PI. 2, Fig. 11 '» 



Massachusetts to North Carolina 



ARCA CAMPECHIENSIS AMERICANA Wood. Ameri- 

 can Ark. Ribs about thirty-five, each with 

 a median impressed line, spaces betv;een 

 ribs deeply cut; anterior end very short 

 and contracted; epidermis blackish brovm; 

 ligament area extremely narrow; umbones 

 anterior and almost touching each other. 



This elongate form is common in 

 Carolina waters. 



PI. g. Fig. 13 



North Carolina to Florida and 



Texas 



ARCA INCONGRUA Say. Shell inequivalve, 

 rather short and much inflated; umbones 

 well separated; ligament area wide and ex- 

 cavated; about twenty broad low ribs upon 

 surface and ten more behind the posterior 

 ridge, these ribs with concentric elevated 

 ridges upon their summits which are less 

 distinct toward the posterior end; ribs 

 clearly visible inside shell and strongest 

 toward the margin; numerous erect, gradu- 

 ated teeth upon hinge margin, becoming 

 smaller toward the center. Length 2 inch- 

 es. 



This very striking shell may read- 

 ily be separated by its curved outer margin, 

 which in shape resembles a weakly-defined 

 letter S, when the shell is viewed toward 

 the posterior end. The plump shape is al- 

 so characteristic. Mr. F. S. Webber, whose 

 specimens are figured, reports beach shells 

 plentiful at Wilbur, south of Daytona Beach, 

 Florida. 



PI. 3, Fig. 2 



Cape Hatteras, North Carolina to 



Texas 



ARCA TRANSVERSA Say. Transverse Ark. Shell 

 transversely oblong, about thirty-five 

 ribs; umbones separated by a long narrow 

 space and situated at one third the length 

 of the hinge margin. Length 1.5 inches. 



Not infrequent near the sands of 

 Nantucket and Martha's Vineyard, also in 

 shallow water off the v;est coast of Flor- 

 ida. 



PI. 4, Fig. 4 



Cape Cod to Key West 



ARCA SECTICOSTATA Reeve. Cut-rib Ark. 



Shell large, somewhat elongated; lombones 

 full and high, situated one fourth dis- 

 tance between ends but closest to anterior 

 end; about thirty-five ribs which are 

 grooved in the middle and at anterior end 

 of shell, posterior ribs rounded, all ex- 

 cept those upon the posterior slope some- 

 what beaded; hinge long and with many low 

 teeth; margin strongly toothed; color 

 white, with brown shaggy epidermis. Length 

 3.5 inches. 



This fine large Area is not rare 

 at Sanibel. A shell similar to this spe- 

 cies is found in the fossil beds west of 

 Palm Beach. 



PI. 3, Fig. 5 



PI. 2, Fig. 10 



North Carolina to Florida and west 



to Texas 



ARCA CHEMNITZI Philippi (A. orbignyi Ko- 

 belt) . Shell a little inequivalve, trian- 

 gular but irregular; very high incurved 

 umbones; about twenty-five nodulous ribs 

 which are often flattened; epidermis in 

 grooves of anterior half inclined to be 

 spiny; hinge short, teeth distinct, inner 

 m£.rgin strongly toothed; color white under 

 the epidermis. Length 1.25 inches. 



PI. 3, Fig. 6 



Florida to Texas and West Indies 



SUBGENUS NAVICULA Blainville 1818 



ARCA OCCIDENTALIS Philippi (A. noae of au- 

 thors) Western Ark. Shell oblong, poste- 

 rior end wider and cut off, solid, inflat- 

 ed; epidermis shaggy; at base a long nar- 

 row gap through which passes the attach- 

 ment byssus; surface plainly ribbed, larg- 

 est ribs in the middle and smaller ones be- 

 tween them; lozenge-shaped area between um- 

 bones covered with a dark colored ligament; 

 pallial line inside often slightly indent- 

 ed. Color brown, ornamented with zebra- 

 like stripes of brown. Length 2.5-3 inch- 

 es. 



PI. 3, Fig. 7 



North Carolina to the Gulf of Mex- 

 ico and West Indies 



"ARCA UMBONATA Lamarck. Beaked Ark. Shell 

 solid, inflated; posterior ridge sharp and 

 with fine radiating ribs in front of it; 

 about eight stronger nodulous ridges behind 

 the posterior ridge; base gaping in front; 



