PANAMIC-PACIFIC PELECYPODA 141 



in the Rio Guapi in southwestern Colombia; it usually occurs where the 

 river water is clear and flows over a rocky bottom, the mussel attached in 

 clusters to small boulders, submerged trunks of trees or nestling in holes 

 along the bank if the wall is rock. The associated mollusks are a small 

 Neritina and Lithococcus (Hydrobiidae). 



Range — Rivers of western Colombia and northwestern Ecuador. Colom- 

 bia: Rio San Juan; Rio Guapi. Ecuador: Rio Verdi; Rio Cayapas, 



Mytilopslg adamsl Morrison and M. zeteki Hertlein and Hanna 



Plate 84, figures 9, 9a 



Two Other species of MytUopsis have been described from the Panamic 

 region but because of the lack of representative material, the status of these 

 two species cannot be analysed at this time. M. adamsi Morrison, 1946, 

 (Smith. Misc. Coll., vol. 106, No. 6, pp. 46, 47, pi. 1, fig. 4) was described 

 from the uppermost end of a fresh-water lagoon on San Jose Island, a 

 member of the Pearl Island group in Panama Bay. Its figure shows an 

 elongated shell, twice as long as high, with a relatively large septum and 

 eroded, deformed beak. Morrison related the species with the Atlantic 

 Coast M. leucophaetus of Conrad. 



Another species of the genus was described by Hertlein and Hanna, 

 as M. zeteki from the Miraflores Locks (Hertlein and Hanna, 1949, 

 Bull. 7, Cahf. Acad. Sciences, vol. 48, pt. 1, pp. 13-18, pi. 1). In a note 

 from Morrison, he stated that he considered M. zeteki close to M. adamsi 

 if not a growth-form of that species. A few specimens of a small MytUopsis 

 have been collected from shell drift from Venado Beach, Panama Canal 

 Zone. These specimens are small, hatchet-shaped, the height about equal 

 to their length, the interior with a small septum, and the surface white or 

 lightly shaded with gray or pale black. In their present form, the Venado 

 shells agree well with young specimens of M. sallei from Colon, and may 

 indicate a Pacific invasion of that Caribbean species. The two West Atlantic 

 MytUopsis are sometimes difficult to separate; M. sallei is generally higher 

 and more hatchet-shaped and the internal apophyses under the septum is 

 smaller, closer to the dorsal wall, whilst M. leucophaetus is narrowly 

 elongated, its apophyses larger, often closer to the middle and hence more 

 prominent. 



Family JULIIDAE (Opistobranchia Sacoglossa) 



Small or minute, bivalved mollusks, greenish in color and living amongst 

 green algae. In the genus Julia, the shell is roughly aviculoid in shape, both 

 valves equal and strongly convex with large, full umbones terminating in 

 small beaks. The higher, longer side is the anterior with a well-rounded end; 

 the shorter side is posterior, pointed at the end with a deep, rounded inden- 

 tation below the beaks resembling a large, sunken lunule. The hinge is heavy, 

 with a large, stout tooth in each valve and which fits into a corresponding 

 socket in the opposite valve. There is a single, large adductor scar in each 

 valve, more or less medially placed and with one or more, smaller accessory 



