424 AXEL A. OLSSON 



lines. Pallial sinus large, open, rounded, extending nearly to the middle of 

 the shell's interior. In the living animal, the siphons are protected by armor 

 as in Mya. 



In typical Platyodan, the shell is relatively large, and attains a length of 

 60 mm. or more. 



Subgenus AUSTROPLATTODON, new subgenus 



Type species P. (Austroplatyodon) australis, new species. Esmeraldas, 

 Ecuador. 



Like Platyodon in general shape and surface sculpture but the shell 

 small, generally less than 6 mm. in length. The anterior side is longer, 

 strongly convex, and expanded, the posterior side much shorter, narrowed. 

 Platyodon (Anstroplatyodon) australis, new species Plate 76, figures 6, 6a 



Similar in shape, sculpture, and hinge to Platyodon cancellata Conrad, 

 from California and Washington but much smaller, the largest specimen 

 before me about 5.7 mm. in length. Valves inequilateral, subequivalve except 

 for some distortion, convex, and relatively thin. Umbones are wide, ending in 

 small, adjacent and usually eroded beaks which are placed near 

 the posterior one-third, the anterior side hence larger, strongly inflated, 

 and widely rounded at the end, the posterior side shorter, much narrowed 

 and terminating in a bluntly rounded, gaping end. The left valve has a 

 wide resilifer or chondrophoral plate, its margin oblique as if cut at an 

 angle. In the right valve, the resilifer is developed as a wide pitlike scar 

 in the roof of the umbonal cavity. There are no hinge teeth. External 

 sculpture of fine, close-set, and quite uniform concentrics. The posterior 

 surface set off sharply from that of the disk by a keeled umbonal ridge, in 

 front of which the surface is somewhat depressed. The texture of the shell 

 is generally so thin as to appear subtranslucent with the external markings 

 showing plainly in the interior. The pallial sinus is usually indistinct but 

 much as in Platyodon, s.s., the impression of the posterior adductor scar 

 large and deep. 



Length 4.7 mm., height 3.1 mm. Esmeraldas, Ecuador. Holotype ANSP 

 218912. 



Probably a nestler in small crevices. 



Range — Ecuador. Ecuador: Esmeraldas. 



Genus SPHENIA Turton, 1822 



Type species by original designation. Gray, 1847. Sphenia binghami 

 Turton. European Seas. 



The shell is small, inequivalve, elongate, usually irregular because of 

 a nestling habit in crevices or in abandoned burrows. Hinge without teeth. 

 Ligament as in Mya, the left valve bearing a large chondrophoral plate. 

 Pallial sinus large, widely rounded at the end. Surface with irregular growth 

 line sculpture. 

 Sphenia fragilis Carpenter Plate 77, figures 9-9b 



Sphaenia fragilis Carpenter, 1857, Cat. Mazatlan Shells, Brit. Mus., pp. 24, 25, No. 35. 



Mazatlan, in the burrows of worms and mollusks in Chama and Spondylus; 



also in dead Balani on Strombus galea. 

 Shell small, white, relatively thin, length seldom over 8.5 mm., irreg- 

 ularly inequivalve, the umbones and beaks placed near the anterior one- 

 third which is also the zone of maximum inflation. Behind the beaks, the 

 posterior side is often narrowed, produced, and of irregular form, its end 

 truncate and gaping; in others, the posterior side is as high as the anterior 



