416 AXEL A. OLSSON 



Length 19.1 mm., height 9.7 mm., diameter of a left valve 4 mm. 



Because of its thin and easily broken shell, this species is seldom seen 

 in collections. My specimens were obtained from beach drift taken at Venado 

 Beach in the Panama Canal Zone. 



Range — Panama. Panama: Panama City, (C. B. Adams). Panama 

 Canal Zone: Venado Beach. 



Subgenus SIACOrLOUIA Pilsbry and Olsson, 1941 



Type species Macoma ecuadoriana Pilsbry and Olsson. Pliocene of 

 Ecuador. 



Shell elongate, nearly equivalve, the left valve a little larger and more 

 convex than the right; its surface covered with earthy granules, heaviest 

 on the posterior area, absent or weaker on the rest of the disk. 



Distinguished from other macomid genera by the heavy granulation 

 of the posterior submargins, suggestive of that of Periploma. 



Psammacoma (Macoploma) medioamericana (Olsson) 



Macoma {Macoploma) medioamericana Olsson, 1942, Bull. Amer. Paleont, vol. 27, No. 

 106, p. 196, pi. 17, fig. 8 Pliocene, Quebrada Penitas, Burica Penn., Costa 

 Rica. — Hertlein and Strong, 1949, Zoologica, vol. 34, pt. 2, p. 93. 



Shell elongated, resembling P. lamproleuca in size and shape but pro- 

 portionately longer and the surface is covered irregularly with earthy 

 granules, heaviest on the posterior area. A specimen from the Gulf of Nicoya, 

 Costa Rica, recorded by Hertlein and Strong has the following measure- 

 ments: length 101 mm., height 54 mm., diameter of both valves together 

 24 mm. 



First found as fossil in the Pliocene of the Burica Peninsula in Costa 

 Rica but since recorded as still living by Hertlein and Strong at several 

 places from Mexico southward to Panama, P. ecuadoriana Pilsbry and 

 Olsson is another fossil species from the Pliocene of Ecuador. 



Range — Gulf of California to Panama. 



Genus PSA3OI0THALIA, new genus 



Type species, Tellina cognata C. B. Adams. 



The shell is subrhomboidal or Psammobia-like in shape, with flattened 

 umbones and the beaks placed a little behind the middle, the two ends of 

 the valves almost alike, except that the shorter posterior side is more 

 depressed and its margin more widely truncated. The hinge is provided 

 with small cardinal teeth, but there are no laterals; the left valve has a 

 single, narrowly bifid, cardinal tooth with a socket on each side: the right 

 valve has two, small, bifid, cardinal teeth. The ligament is external, at- 

 tached to a long, narrow, grooved scar along the upper side of a nymphal 

 ridge. The adductor scars are large and nearly equal, placed high in the 

 interior of the valve. The pallial sinus is deep, its upper limb almost reach- 

 ing across to the anterior adductor scar, rounded at its end and joined 

 with the pallial line below for about half its length. The external surface 

 is usually smooth and polished, marked lightly with concentric lines of 

 growth and diagonally across these a set of incised lines {Scissula-like) 



