MOLLUSC A OF PORTO RICO. 
471 
Section LITHOPHAGA Bolten, s. s. 
Lithophaga antillarum d’Orbigny. 
Lithodomu s antillarum d’Orbigny, Moll. Cubana, IT, 1S47, p. 332, pi. \ win, figs. 12, 13. 
Shell elongated, rounded in front and behind, wedge-shaped when viewed from above; the beaks 
placed at the extreme upper anterior border; surface sculptured with concentric furrows, especially 
the posterior part; the anterior and anterior basal portion having fine vertical ribs which fade out 
at the extreme anterior end of the shell; epidermis rich dark chestnut, becoming almost black at 
the posterior end and anterior base. 
Length, 30; height, 10; diameter, 8 mm. 
Puerto Real; Guanica; Arroyo, Porto Rico. 
Section DIBERUS Dali. 
Lithophaga bisulcata d’Orbigny. 
Lithndomus bisulcaius d’Orbigny, Moll. Cubana, II, p. 133, pi. xxvin, figs. 14-16, 1845. 
Shell elongated, subcylindrical, the anterior and posterior ends rounded, the latter with two faint, 
radiating suleations, one above and the other below the low rounded posterior ridge; growth lines 
somewhat impressed, especially on the posterior portion, otherwise the shell itself is smooth and 
shining, the epidermis being a light chestnut color; beaks low, anterior, but not at the extreme end 
of the shell. The greater part of the surface of the shell is covered with a calcareous incrustation, 
which is thin in front and thick behind, and is especially strong on the posterior ridge; its surface is 
partly covered with fine wrinkles or corrugations, and it is pointed behind. 
Length over all, 30; of shell, 25; height, 10; diameter, 8 mm. 
Guanica; Puerto Real; Arroyo, Porto Rico. 
Genus CRENELLA Brown, 1827. 
Shell oval or rhomboidal, with a thick epidermis; surface sculptured with longitudinal anu 
concentric strife; beaks straight; ligament small; hinge area crenulate. 
Crenella divaricata d’Orbigny. 
Nuculocardia divaricata d’Orbigny, Moll. Cubana, n, p. 311, pi. xxvii, figs. 56-59, 1845. 
Shell small, equilateral, solid, inflated, with an elliptical outline; umbonal region high, the beaks 
straight, curved in to the hinge line; surface sculptured by delicate radial ribs, which are somewhat 
divaricate along a line from the beaks to the middle base; near the anterior and posterior there 
is a curved rib, and from this the ribs in front and behind are sharply divaricate; there is also 
delicate concentric sculpture, which causes the ribs to appear as if slightly beaded; inner edge eremite 
throughout. Color brownish yellow. 
Length, 3; height, 3.5; diameter, 3 mm. 
Mayaguez, Porto Rico, several worn valves. 
Genus MODIOLARIA Beck. 1840. 
Shell rhomboid, inflated, generally with three areas on the disk, the two at the ends radially 
sculptured, that of the center smooth or only feebly radially sculptured; sometimes the surface is 
radiately ribbed throughout or it may be smooth; beaks incurved, placed near the anterior end. The 
genus is close to Crenella and differs from it principally in anatomical characters. 
Modiolaria lateralis Say. 
Mytilus lateralis Say, Journ. Acad. Nat. Sci. Phila., ii, p. 264, 1822. 
Shell small, long- rhomboid, inflated, subsolid; umbonal region full; beaks curved forward, pro- 
jecting slightly in front of the anterior end of the shell; surface sculptured with fine, concentric growth 
lines throughout, anterior and posterior areas sculptured with fine, beaded ribs, the posterior area wide 
and including the rounded posterior ridge; the anterior area narrow; central area slightly depressed 
and showing only concentric striae; inner edge crenate, except at the ligamental and central areas. 
Color greenish or reddish, often marbled with brown. 
Length, 6; height, 3.5; diameter, 3 mm. 
San Juan Harbor; Mayaguez, Porto Rico; Vieques, and off Vieques. 
