460 MEMOIRS OF THE CARNEGIE MUSEUM. 



^ 



standing vertically to the edge of the teeth. Similar ridges may be present on 

 the pseudocardinals. 



In the shape of the muscle-scars on the inside of the shell, there exists no 

 great difference between the Hyriince and Muk'Uncc. The general rule is in the 

 Hyriince that the upper anterior retractor-scar is distinctly separated from the ante- 

 rior adductor impression, while the lower anterior retractor-scar is confluent with the 

 latter. Very rarely the lower anterior retractor-scar is isolated. (In the Mutelince 

 either all three scars are confluent, or the lower anterior retractor-scar is isolated; 

 the upper anterior retractor-scar is hardly ever isolated) . The posterior muscle-scars 

 are confluent, the posterior retractor-scar forming a small process at the upper margin 

 of the adductor-scar. The muscle-scars may be deeper or shallower, the upper an- 

 terior retractor-scar generally is rounded, small, and remarkably deep. 



Dorsal muscle-scars are present in the Hyriince (mostly absent in the Mutelince) ; 

 they are variable in number, generally only a few, and are located in the bottom 

 of the shallow beak-cavity, forming an irregular row parallel to the hinge-line, or 

 somewhat oblique. In some forms with deep beak-cavities, these scars are situated 

 on the inner side of the hinge-plate (behind the pseudocardinals and the inter- 

 dentum) . 



The line of the mantle-impression is always simple, without a sinus poster- 

 iorly. The ligamental sinus lies on the margin of the shell behind the ligament, 

 over the jjosterior part of the lateral teeth (rarely more in front), and is always 

 small and shallow (longer than deep). (In the Mutelince, the ligamental sinus is 

 generally much deeper, with a sharp lower point.) 



The characters of the soft parts of the Hyriince have been pointed out above. 



The Genera of the Hyriin^. 



The genera of the Hyriince have been hitherto differentiated according to 

 the shape of the shell, the character of the beak-sculpture, and the character and 

 the sculpture of the hinge-teeth. According to Simpson (1914 pp. 1194 ff.) there 

 are seven of them in South America : Tetraplodon Spix {recte Castalia Lamarck) ; 

 Castalina Von Ihering; Castaliclla Simpson; Callonaia Simpson; Hyria Lamarck; 

 Prisodon Schumacher; Diplodon Spix. 



In addition, there are species belonging to this group in Australia (and probably 

 also in New Zealand), which have been placed by Simpson in Diplodon, but have 

 been separated by him under the subgenus Hyridella Swainson. I have shown 

 (Ortmann, 1912) that the type of HyrideUci {Unio austirdis Lamarck) actually is 

 closely allied to Diplodon, but that probably it is better to regard Hyridella as a 

 genus by itself. 



