JUKES-BEOWNE : ON THE MYTILID^. 221 



Genus Modiola, Lamk. 



Section Eumodiola (von Ihering). Shell obli(iuely oblong, more or 

 less expanded on the posterior side, and inflated along a central 

 oblique line from the umbo to the posterior side of the ventral border. 

 Ex. M. modiolm, (Linn.). 



Section Amygdahim (Mcgerle). Shell elongate, narrow, not 

 inflated, smooth, and polished. Ex. M. arborescens, (Chemn.). 



Subgenus Adula (H. & A. Adams). Shell elongate, transverse, 

 straight, and subcylindrical ; umbones between centre and the anterior 

 end. Ex. A. soleniformis, (d'Orb.). 



Subgenus Myrina (H. & A. Adams). Shell oblong, transverse, not 

 oblique, and nearly equilateral. Ex. M. pelagica, (Forbes). 



Genus Lithodomus, Cuvier. 



Shell long, narrow, and subcylindrical. Umbones at or near the 

 anterior end, which is rounded and inflated. Posterior end generally 

 wedge-shaped, but sometimes cylindrical and tapering. Shell surface 

 smooth or wrinkled. Hinge-line straight, without crenulations. 

 Margins of shell quite smooth. Adductor scars oval and nearly equal ; 

 byssal scars oval and small. Ex. L. lithophagus, (Linn.). 



Subgenus Botula (Morch). Shell oblong, subrhomboid, umbones 

 terminal, prominent, and spirally curved. Ex. B. cinnamomea, (Lamk.). 



We now come to the species which possess denticles or crenulations 

 on the hinge-line behind the ligament, and these 1 propose to group 

 in a separate genus, because I regard this character of generic 

 importance, and of much greater value than the relative position 

 of the umbones, which has hitherto been regarded as the only real 

 distinction between Mytihis and Modiola. 



I agree with von Ihering in grouping together the shells with 

 subterminal umbones, which Morch made a subgenus of Mytilus, 

 under the name of Surmomya, and the shells for which Swainsoa 

 proposed the name of ' Brachidontes^^ generally regarded as a sub- 

 genus of Modiola ; but I cannot follow him in transferring the typical 

 species of Brachidontes to Hormomya, and in retaining the former name 

 for Modiola demissa. 



The name 'Brachidontes^ (Swainson, 1840) has priority over that 

 of Hormomya (Morch, 1853), and Swainson's type is Modiola sulcata, 

 Lamk. ; hence Swainson's name, or some amended form of it, must 

 be taken as the name of the new genus, though Hormomya can be 

 used as a subgeneric name for the striated mj^tiloid forms like 

 M. exustus, Linn., and M. Dominyensis, Lamk. From these latter 

 the little group for which the Marquis di Monterosato proposed 

 the name Mytilaster in 1884, differs only in the character of the 

 external markings, but it may be convenient to retain his name and 

 to regard the assemblage as a second subgenus. 



The construction of the name Brachidontes is obviously very bad ; 

 one supposes that Swainson meant to convey the meaning of short 

 teeth J and Fischer altered it to Brachydontes^ but that is little better, 



