200 AXEL A. OLSSON 



II. Shell rounded, subci'rcular and convex or plump. Surface sculptured 

 with coarse concentrics, often with coarse nodes or pustules most 

 heavily developed on the anterior slope. Ligament deeply immersed, 

 the margins of the valve typically rising well above the nymphal area. 



Genus Phlyctiderma 



Key to Panamic species of the Diplodontidae 



I. Surface covered with fine, growth-like concentrics, often irregular, 

 smooth or minutely punctate (not coarsely nodose or granulose). Liga- 

 ment external but not immersed into the hinge plate {Diflodonta.) 



A. Shell rounded, globose and plump. 

 Aa. Without submicroscopic puntation. 



1. Shell relatively large (length 37 mm.), nearly circular in shape, the 

 umbones low. 



D. suprema 



2. Shell smaller (length about 18 mm.), venerid in shape and with the 

 umbones and beaks rising well above the hinge line. 



D. inezensis 



Ab. Minutely punctate, especially over the surface of the umbones. 



3. Shell relatively small (length 17 mm. or less), rounded with the um- 

 bones and beaks decidely anterior in position. 



D. discrepans 



B. Shell with a round or subquadrate outline, its surface depressed or 

 slightly convex. 



4. Valves subquadrate in shape, the anterior side narrowed. 



D. suhquadrata 



5. Valves subcircular, with low umbones and beaks, slightly anterior in 

 position. Cyclinella-Wke. Length about 20 mm. 



D. tellinoides 



6. Less circular in shape, the umbones high and narrow, projecting above 

 the hinge hne. Length about 19 mm. 



D. cornea 



7. Shell quite small, length about 11 mm. Axis of shell obliquely inclined. 



D. obliqua 

 [I, Shell rounded and convex, its surface plainly sculptured with concentrics 

 or with a scattering of nodes and pustules. Ligament external but 

 deeper, partly immersed in the hinge plate (Phlyctidenna). 



C. Sculpture plain, without nodes or pustules. 



8. Subcircular, convex, length 18 mm. Umbones not prominently elevated. 



P. orbella 



D. Sculpture formed by strong, cordlike concentrics dividing into rows 

 of beadlike nodes or isolated pustules. 



9. Shell relatively large (length about 20 mm.). The ligamental scar deeply 

 impressed into the hinge plate. 



P. semiaspera 



10. Like semiaspera but smaller and with the nodes confined mainly to 

 the anterior submargins. 



P. insula 



11. Shell small with wide umbones which rise high above the hinge line, 

 the anterior side extended. Surface nodes small, strongest on the an- 

 terior submargin. 



P. elenensis 



