MOLLUSCA OF PORTO RICO. 
469 
Genus PODODESMUS Philippi, 1837. 
Shell with the appearance of Anomia, but the left valve with only two muscular impressions. 
Pododesmus rudis Broderip. 
Placananomia rudis Broderip, P. Z. S., 1834, p. 2. 
Pldcnnannmia rudis Reeve, Coneh.Ieon., xi, pi. i, fig. 2, 1859. 
Shell very irregular, compressed, suborbienlar, oval, oblong or somewhat elongate, moderately 
solid; lower valve normally flatter than the upper; byssal opening round oval or closed in old 
specimens; surface wide, with irregular, broken, rather fine ridges crossed by scaly or foliaceous 
growth lines; within shining, the lower valve generally having a brown blotch in its center. Color 
whitish or greenish. 
Length, 40; height, 40; diameter, fi mm. Length of another specimen, 30; height, 35; diameter, 
4 mm. 
Aguadilla; San Juan Harbor; off Point. Melones, Porto Rico. 
Superfamily MYTILACEA. 
Family MYT1L1D/E. 
Genus MYTILUS (Linnaeus, 1758) Bolten, 1798. 
Section H0RM0MYA Morch, 1853. 
Shell wedge-shaped, rounded behind, with terminal beaks, plicately sculptured; hinge teeth 
minute or obsolete; pedal impressions two in each valve; anterior adductor impression small, placed 
under the beaks; posterior adductor elongated, placed at the posterior end of the. shell. 
Mytilus hamatus Say. 
Mytilus hamatus Say, Journ. Acad. Nat. Sei. Pliila., n, p.265, 1822; Binney’s Say, pp. 91,204, pi. l, fig. 18. 
Shell irregularly triangular, rather solid, elongated and generally falcate, the anterior side being 
usually incurved, the posterior side angular where it joins the hinge; there is a more or less prominent 
rounded ridge back of the anterior part of the shell, and the radiating ribs with which the whole shell 
is sculptured spread from this ridge, curving to the front and the back of the shell; these ribs are 
numerous and somewhat bifurcate; they are finely beaded and often wavy; inner border of the shell 
erenulate. The thick epidermis is blackish or brownish, sometimes tinted green and sometimes a 
little tawny; the interior is usually dark violet, with a lighter border. 
Length, 30; height, 50; diameter, 19 mm. 
San Juan, Porto Rico; Ensenada Honda, Culebra; Sail rock, between Culebra and St. Thomas. 
These specimens are more delicate and less curved than the ordinary manifestation of this species. 
Mytilus exustus Linnaeus. 
Mytilus exustus Linnssus, Syst. Nat., eel. X, p. 705, 1758. 
Mytilus striatulus Schroter, Einleitung. nr, p. 449, pi. ix, fig. 10. 
Shell small, rather solid, inflated, irregularly triangular, somewhat elongated; beaks curved 
inward and forward; there is a high, rounded ridge beginning at the beaks, then curving backward, 
then downward and forward to the posterior base of the shell; the surface is sculptured with numerous 
curved ribs, which follow the course of the ridge and curve each way from it to the anterior and 
posterior sides; these ribs are beaded and somewhat bifurcated; at the byssal region in front there is 
a somewhat smooth area; the growth lines are often deeply impressed at intervals. Color brownish, 
blackish and tawnv, often somewhat clouded. 
Length, 10; height, 18; diameter, 10 mm. 
Puerto Real; Hucares; Ponce; San Juan, Mavaguez, Porto Rico; Ensenada Honda, Culebra. 
Genus MODIOLUS Lamarck, 1799. 
Shell much as in Mytilus, but the beaks are not terminal; there are no hinge teeth; the epidermis 
is hirsute, and the animals generally nestle in a mass of byssal fibers. The valves have usually, if 
sculptured, a smoother central area. 
