118 AXEL A. OLSSON 



anterior one small and rounded, the posterior longer, pipe-shaped connected 

 with the retractor scar. The pallial line not enlarged. Surface with ribbed 

 sculpture. 



There is some question as to the identification of Lamarck's M. sulcatus 

 and whether it represents an Atlantic or Indo-Pacific species. The above 

 description is based largely on M. citrinus Roeding from Florida. (See Olsson 

 and Harbison, 1953, Mon. No. 8, Acad. Nat. Sci. Philadelphia, pp. 61, 62.) 

 Brachidontes citrinus (Roeding) {:=zArca modiolus Linne, 1767), Plate 

 12, figure 3. 



Genus HORMOMTA Morch, 1853 



Type species by subsequent designation, Jukes-Brown, 1905, Mytilus 

 exustus Linne. 



Shell small or medium-sized, mytiliform, with the small beaks near but 

 not quite terminal, the valves vaulted, convex or angulated along the long, 

 umbonal axis, often deeply impressed on the ventral side. Dorsal margin 

 straight or slightly convex, curving into the posterior margin, the ligament 

 and its scar narrow and lying along nearly its whole length. Posterior and 

 vientral margins strongly crenulated by the external sculpture, the 

 short, anterior margin with similar small crenulations and not the larger 

 and stronger dysodont teeth. Anterior adductor scar relatively large, ovate, 

 the pallial line distinct, the posterior adductor scar, large, pipe-shaped and 

 often covering a large part of the inner posterior surface. External surface 

 ribbed, sometimes much reduced or subobsolete over the impressed ventral 

 portion. 



Like Brachidontes, but differs by its longer ligament which extends 

 along most of the dorsal margin; the dorsal margin not normally crenulated 

 above the ligament; also by the larger posterior adductor scar. 



Honnomya exustns (Llnn6) Plate 12, figure 2 



Genus SCOLDtYTILUS, new genus 



Type species Modiolus (Brachydontes) playasensis Pilsbry and Olsson. 



Shell small, mytiliform or modioliform, the beaks plated almost termin- 

 al, the valves elongated with the longer, oblique axis lying along the con- 

 vex or sharply arched umbonal slope, the ventral side commonly flattened 

 or deeply impressed. The surface sculpture is formed by radial riblets which 

 may be weak or coarse. The posterior adductor scar is fused with that of 

 the retractor and with a part of the pallial impression forming a single, 

 large, lobate-shaped or ribbon-like band within the posterior margin. Liga- 

 ment, as in Hormomya, is nearly as long as the posterior-dorsal margin. 

 Hinge with one to three, fairly large, strong, dysodont teeth under the 

 beak. The valve margin is crenulated throughout except for a short space 

 along the impressed ventral side where the fluting of the riblets is weak 

 or obsolete. 



A group of small species combining some of the characters of Brachi- 

 dontes and Hormomya but sufficiently distinct to warrant generic separa- 

 tion. Two subgenera may be recognized. 



