54 AXEL A. OLSSON 



Key to genera of Panamic Nuculidae 



I. Surface smooth or with radially cancellate sculpture. 



Genus Nucula 

 II. Surface with divaricate sculpture. 



Genus Acila 

 Genus NUCUIiA Lamarck, 1799 

 Type species by monotypy, Area nucleus Linne. 



Recent in the European Seas 



Shell generally small, with closed margins, not gaping, ovate-trigonal to 

 subcircular, with small, appressed, opisthogyrate beaks. Interior white and 

 usually brilliantly pearly. Surface smooth but in some cases showing faint 

 to quite strong, subcutaneous radial lineation, stronger ventrallv and giving 

 rise to a pectinate or crenulate margin. Escutcheonal area usually somewhat 

 elevated and pouting. Hinge taxodont, with two series of crowded chevron- 

 shaped teeth, the anterior row longer and arched, the posterior row shorter 

 and with fewer teeth. Chondrophore narrow, inclined obliquely foward. 

 Pallial line simple. 



About 13 species of Nucula have been recorded from the Pacific coasts 

 of northern South America, Central America, and Mexico or its adjacent 

 waters. About half of these species are abyssal and hence do not properly 

 belong to the Panamic-Pacific fauna. Reference to these off-shore species 

 will be found in Hertlein and Strong. 



The genus Nucula is commonly divided into two sections or subgenera 

 on basis of a smooth or crenulate ventral margin. 



Subgenus KTCULA s.s. 



Inner layer of shell with radially ribbed structure which may show on 

 the surface either strongly or weakly as fine radial striations. Inner ventral 

 margin of valves crenulate or denticulate. 



Subgenus EJOfUCULA Iredale, 1931 



Inner layer of shell without a radial structure and the ventral margin 

 of valves without crenulations. Surface smooth except for growth in- 

 crementals. 



Key to species of Nucula 



I. Inner margin of valves crenulated; the finely ribbed structure of the 

 inner layer transmitted to the surface and showing plainly or in- 

 distinctly. 



Subgenus Nucula, s.s. 



A. Shell very large (30 mm. or more in length). 



1. Surface smooth or finely radially striate, the umbones more strongly 

 sculptured with narrow, wavelike, concentric riblets cut by fine radial 

 striation. 



A^. iphigenia 



B. Shell smaller. 



a. Surface smooth or finely radially striated. Inner margin of valves 

 finely crenulated. 



2 „ N. declivis and subspecies 



b. Surface with stronger sculpture, cancellated with both concentric and 

 radial riblets. Inner margin coarsely crenulated. 



