410 AXEL A. OLSSON 



without any teeth or interlocking sockets. The pallial sinus is usually large, 

 entire, its end rounded, and its lower limb partly joined with the pallial 

 line. Margins of the valves smooth and without crenulations. 



The typical species of the genus Macoma are cold-water forms. 

 In that group, the surface of the valves is usually chalky and covered with 

 a coarse dark-colored periostracum. Species from warmer waters have a 

 thinner and lighter-colored periostracum, the surface beneath smooth, 

 polished, and often a brilliant white. The ligament, as in the Tellinas proper, 

 is external but in some genera as Florimetis, Psammotreta and the like, it 

 is partly immersed or lies more deeply inset in the hinge plate; in such 

 cases, the ligamental scar is often much reduced in size and has a triangular 

 shape, closely appressed against the posterior cardinal tooth. The extent 

 of the posterior flexure varies between different groups; it is wanting com- 

 pletely in some or so slight as to be hardly noticeable; in others, it is 

 sharp and angular, and the posterior-dorsal slope may be expanded into a 

 large winglike area as in Florimetis. 



Genus FLORDIETIS Olsson and Harbison, 1953 



Type species by original designation, Tellina intastriata Say. Florida. 



Shell broadly subovate, inequivalve with a wide, depressed or winglike 

 posterior area, which in the right valve is set off by a fold or angled keel 

 extencing from the beak to the posterior extremity. Both valves are strongly 

 convex along the anterior umbonal slope, the right valve being impressed 

 just behind it or across the middle. In the adult, the adductor scars are 

 plainly marked, the anterior one being narrowly elongate or lucinoid with 

 the pallial line attached to its lower end; the posterior scar rounded and 

 placed quite low. Pallial sinus large and deep, reaching into the middle 

 of the shell cavity and confluent below with the pallial line by more than 

 half of its length. Hinge bearing small cardinal teeth, the left anterior and 

 the right posterior tooth are bifid; there are no lateral teeth. Ligament 

 external with a large, nymphal ridge. Outer surface white, the sculpture 

 produced by growth incrementals, usually coarse over the posterior areas. 



This group of American species has generally been known by the generic 

 names of Metis or Apolymetis, the type of which is a little known East 

 Indian species not closely related to our American shells. (See Olsson and 

 Harbison, 1953, Mon. 8, Acad. Nat. Sci. Philadelphia, p. 129). 



Flurimetls cognata (Pilsbry and Vanatta) Plate 67. fgures 2-2d; 



Plate 68, figure 10 



Lutricola cognata Pilsbry and Vanatta, 1902, Proc. Washington Acad. Sci., vol. 4, p. 

 556, pi. 35, fig. 5 "Tagus Cove, Alb-marle." Galapagos Islands. 



Apolymetis cognata (Pilsbry and Vanatta), Pilsbry and Lowe, 1932, Proc. Acad. Nat. 

 Sci. Philadelphia, vol. 84, pp. 96 (in text), 133. — Hertlein and Strong, 1949, 

 Zooiogica, vol. 34, pt. 2, pp. 93, 94. 



Apolymetis excavata (Sowerby), Pilsbry and Olsson, 1941, Proc. Acad. Nat. Sci. Phil- 

 adelphia, vol. 93, p. 70. Not Tellina excavata Sowerby, 1867 (=^P. dombei 

 (Hanley).) 



Shell large, with a length to about 84 mm., subsolid. Umbones and 

 beaks are nearly median, the anterior side a little longer, the posterior side 

 deeply flexed, producing a large, winglike posterior area. Valves roundly 



