NO. 1 SOOT-RYEN : THE FAMILY MYTILIDAE 37 



posterior part of the ventral margin and the posterior and dorsal margins 

 are crenulate, with some stronger crenulations behind the ligament. The 

 posterior retractors are not fastened to the dorsal part of the adductor, 

 but slightly below, so the dorsal margin of the adductor is higher than 

 the retractors. This is not always discernible from the muscle scars. 



The radial sculpture on the ventral part of the shell below the strong 

 keel is not simply bifurcating, but usually has four or five separate lines 

 arising from one radiating line. This is best seen in the oldest parts of the 

 shell, and when properly observed is a fairly good character for separating 

 species of Hormomya from Brachidontes. The same pattern of the radiat- 

 ing sculpture is also found in Septifer. The posterior part of the mantle 

 margins is furnished with small, apparently very contractile, papillae. 



If Hormomya is restricted to species with anterior umbones, strong 

 umbonal keel, unilateral bifurcating lines or ribs on the ventral part, and 

 the posterior retractors placed slightly below the dorsal margin of the 

 posterior adductor, there seem to be only two west American species 

 referable to this genus. Sometimes more aberrant specimens with appar- 

 ently subterminal umbones and a curved anterior margin may be very 

 difficult to place in the correct group, but usually the distinct umbonal 

 keel separating a coarsely sculptured dorsal part from the finer sculptured 

 ventral part, shows that the specimen belongs to Hormomya. 

 Key to the west American species : 



The radiating ribs at least partly ornamented 

 with distinct round granules, older specimens 



with very curved and inflated anterior part granulata (Hanley) 



The radiating ribs more or less smooth or fur- 

 nished with transverse ridges, dorsal ribs usu- 

 ally much coarser than the ventral ones adamsiana (Dunker) 



Hormomya adamsiana (Dunker) 1856 

 Plate 3, fig. 11 ; text-figs. 22, 25, 31 



Mytilus adamsianus Dunker, Proc. Zool. Soc. London, vol. 24, 1856, 



p. 360. 

 Syn.: Mytilus bifurcatus auct. 



Mytilus stearnsi Pilsbry and Raymond 1898. 

 Holotype: British Museum. 

 Type loc: Isthmus of Panama. 



Remarks: The species to which Dunker's name adamsiana is applied is 

 extremely variable and may perhaps constitute a mixture of two or more 

 species. It has been impossible to solve the question with the material at 



