JUKES-BKOAV^'E : ON THE MYTILIDJE. 223 



Muscular sears similar to those of Eamijtilus or Modiola. Type, 

 C. suicatus, Lamk. (= citrinus, Chemn.).' 



Subgenera. 



Hormomya (Morch). Shell finely ribhed ; umhones subterminal ; 

 anterior side straight or incurved. Type, B. {Hormomya) exustus, 

 (Linn.). 



Mytilaster, Monterosato. Shell mytiloid, with nearly terminal 

 umhones, either with a suj^erficially crinkled surface or marked only 

 by concentric lines of growth. Type, 31. lineatus, (Gmelin), 



Septifer, Reckiz. Shell similar to that of Hormomya, except that 

 the anterior adductor is fixed to a plate which forms a triangular 

 septum at the anterior end of each valve. Type, S. bilocularis, (Linn.). 



Ischadium, subgen. nov. Shell oblong or pyriform in outline, 

 sculptured all over with strong raised divaricating ribs. Umhones 

 slightly divergent ; anterior riblets well-marked, and corresponding 

 with a variable number of dysodont teeth. Ligament long, without 

 crenulations behind it. Anterior adductor scar absent, and anterior 

 byssal scar small. Posterior byssal scar large and broadly united 

 to that of the posterior adductor. Type, /. hamatum (Say). 



It may be useful to enumerate some of the species which may 

 be referred to each of the first four groups. To Brachyodontes, s.s., 

 the following are referable : — 



B. suicatus, Lamk. B. ustidatus, Lamk. 



B. Menkeanus, Reeve. B. Capensis, Krauss. 



B. purpuratiis, Lamk., B. striatulus, Hanley. 



and ? var. ovalis, Lamk. B. Senhausi, Reeve. 



To Hormomya the following species are referable : — 



B. (H) hirsuiiis, Lamk. B. (H.) subramosiis, Hanley. 



B. (H) exustus, Linn. B. (H.) rostratus, Dkr. (in Rve.). 



B. (H.) Bomingensis, Lamk. B. {H.) tenuistriatus,'iykv.{]i\Q.). 



B. [H.) cubitus, Say. B. (H.) curvatus, Dkr. 



B. (H.) Adamsianus, Dunker. B. {H.) granulatus, Hanley. 



B. (H.) Lavalleanus, d'Orb. B. {H.) crebristriatus. Say. 



B. {H.) Barwinianus, d'Orb. B. {H.) Charpentieri, Dkr. 



To Mytilaster the following species belong : — 



M. lineatus, Gmel. M. hybridus, Monterosato. 



M. minimus, Poli. M. Marioni, Locard. 



M. crispus, Cantr. M. Solisianus, d'Orb, 

 M. lacustris, Costa. (= ? J/, exiguus, Dkr.). 



Genus Modiolaeia, Beck. 



I have not paid any special attention to the genus Modiolaria, 

 which is in many respects very similar to Modiola, but the animal 



1 It is a question whether this species will not now have to be called Brachyodontes 

 modiolus, as it is certainly the Area modiolus of Linnseus (1767). 



