'f 



588 MEMOIRS OF THE CARNEGIE MUSEUM. 



It is seen at a glance that these three sections are not uniformly well defined. 

 Virgula, indeed, is shari)ly separated from the rest. Styganodon is well character- 

 ized by the epidermis ; but unfortunately the type of the genus {Anondontites crispata) 

 undoubtedly belongs to Styganodon, having an epidermis (thick, dark, rough, 

 sombre-colored) which represents an extreme development of the Styganodon- 

 structurc; in other characters also, A. crispata is closely allied to A. tenebricosa,-'' the 

 type of Styganodon. 



It is clear that, on the one hand, Anodontites {sensu strictiore) must be used for 

 crispata, and, on the other hand, that Styganodon is a synonym of this, the type 

 of the latter being closely related to crispata. This necessitates a re-arrangement 

 of the sections, and a revision of their nomenclature.. Although I have good 

 material representing the genus, it is impossible for me to attempt a final classifi- 

 cation, and the one given below is primarily adapted to the material at hand. An 

 attempt is made to preserve Simpson's groups as far as possible. 



Geographical distribution: The genus is widely distributed over South America 

 east of the Cordilleras from Patagonia to the Caribbean Sea, being found also in the 

 northern parts, where Diplodon and the Hyriince in general are rare, or absent. 

 West of the Cordilleras it is generally missing, but it has been reported from that 

 side in Ecuador (the fact, however, requires confirmation). In addition the genus 

 has extended its range northward into Central America and Mexico, where it 

 is found in both the Pacific and Atlantic drainages. 



Key to the Groups of Anodontites. 

 ai. Shell not greatly elongated, nor jjointed behind, without sharp posterior ridge. Posterior retractor- 

 scar connected with adiluctor-scar Subgenus Anodontites s. s. 



61. Epidermis dull, densely wrinkleil. 



C:. Shell more or less elongated, but not distinctly ol)liciue and subcircular. 



di. Shell subtrapezoidal, somewhat elongated. Lower margin straight or concave. 



Group of A. crispata. 

 (!■•.■ Shell subovate or subelliptical, rather short. Lower margin convex. 



Group of .1. obtusa. 



C2. Shell distinctly oblitiuc, short and high, nearly subcircular Group of A. trapezea. 



bo. Epidermis more or less shiuing, wrinkles only partially devclo|)ed or absent. Shell straight or 

 oblique, often distinctly s(j. 

 Ci. Shell not very oblique; straight, rather elongatetl, more or less pointed behind. Prismatic 



border narrow, of ecpial width Group of .1. tn'gona. 



Cn. Shell strongly oblique, not much elongated, not, or very little, pointed behind. 

 di. Shell subovate or subrotund. Prismatic border wide, width unequal. 



Group of .1. j)(il(i()onirn. 



=' Another species, A. napoensis, allied to crispata, has been placed in Styganodon by Haas (1910, 

 pp. 32, 55). 



