PANAMIC-PACIFIC PELECYPODA 57 



Shell ovate, inequilateral, the opistogyrate beaks placed nearer the 

 posterior end, the posterior end hence shorter, its margin subtruncated. 

 Anterior side usually much longer, its dorsal margin gently arched, narrowly 

 rounded at the end. Surface smooth, the lines of growth indistinct, por- 

 cellaneously white. Inner ventral margin smooth. Hinge with the posterior 

 set of teeth shorter and fewer in number than the anterior. Chondrophore 

 narrow and strongly oblique. 



Ifucnla (Ennucnla) colombiana Dall Plate 1, figures 3, 3a 



Nucula colombiana Dall, 1908, Bull. Mus. Comp. Zool., vol. 43, No. 6, p. 371. — Hertlein 

 and Strong, 1940, Zoologica, vol. 25, pt. 4, p. 385. 



Shell small (length 4 to 5 mm.), subsolid, inequilateral, the posterior 

 side short, sharply descending, subtruncate and with the margin pouting 

 a little in the middle of the flattened area. Anterior side longer, its dorsal 

 margin somewhat arched, slightly narrowed and rounded towards the end. 

 Umbone wide. Surface porcellaneous white, shiny, very smooth except for 

 one or more shallow, concentric undulations, the growth lines not visible. 

 Interior of shell pearly and with the ventral margin of the valve smooth. 

 Hinge with about seven teeth in the posterior series and about 14 in the 

 anterior. 



Length 4.5 mm., height 3 mm., max. diameter 2.2 mm. 



This is a small, simple species dredged in Panama Bay and off Manta, 

 Ecuador, in waters ranging from 29 to 41 fathoms. It resembles Nucula 

 declivis but its ventral margin is smooth. Dall recorded this species off 

 the coast of Chile in waters as deep as 401 fathoms. 



Range — From Panama southward. Panama: Panama Bay. Ecuador: off 

 Manta. 



Genus ACILA H. and A. Adams, 1858 



Type species by subsequent designation, Stoliczka, 1871, Nucula divarv- 

 cata Hinds. 



Like Nucula but often larger (max. length about 50 mm.), the surface 

 sculptured with a characteristic pattern of parallel divaricating and more 

 or less radial riblets which appear as if stacked or packed close together the 

 main line of divarication extending from the umbo across the middle of the 

 shell disk to the ventral margin, the peaks of their inverted V's forming 

 a line of sharp, acute angles pointing towards the beak. A secondary line 

 of reversed divarication may be developed along the posterior rostral side 

 which often forms a shallow sinus ending in a slight bulge or pout at the 

 margin. 



Acila (Acila) diyaricata burica Olsson Plate 17, figures 9, 9a 



Acila isthmlca burica Olsson, 1942, Bull. Amer. Paleont, vol. 27, No. 106, pp. 177, 178, 

 pi. 1, figs. 2, 6, 8, 9. 

 Acila has not been reported as living in Panamic waters, however a 

 large species is plentiful as fossil in the Pleistocene-Pliocene beds along the 

 Burica Peninsula of southwestern Panama. Some of the fossils bear a strik- 

 ing resemblance to the figures of A. divaricata submirabths Makiyama 

 (Schenck, 1936, Geol. Soc. America, Special Papers, No. 4, pp. 88, 89, pi. 14, 

 figs. 8, 11) from the upper Pliocene of Sasage, Kazusa, Japan, but also 

 known to be living off Japan and the Celebes. It seems possible that A. 

 divaricata burica may still be living in Panamic waters. 



