JUKES-BEOWJfE : ON THE MTTILTD^. 217 



adductor were characters that marked off a certain group of species,^ 

 and that this assemblage included the species which had been separated 

 by Morch in 1853, as a subgenus of Mytilus, under the name of 

 Chloromya?- Morch gave no diagnosis, merely enumerating the species 

 perna, Linn., Africanus, Chemn., variegaUis, Chemn., smaragdinus, 

 Chemn., and latus, Lamk. Von Ihering established the subgenus on 

 the characters above mentioned, and made M. perna the type, but 

 did not notice the teeth, which, as I have shown, are also distinctive 

 of this group. 



There is another group of Mytilus which is also generally mono- 

 myarian by the absence of the anterior adductor, though the arrange- 

 ment of the byssal muscles is the same as in M. edulis. This is the 

 strongly ribbed section typified by M. Magellanicus, and separated 

 by Morch under the name of Aulacomya. As von Ihering remarks, 

 the want of an anterior adductor is not in itself a very important 

 character, and he states that this muscle is really always present in 

 young 31. Magellanicus, though it disappears in older specimens.^ 

 I^evertheless, when its loss is associated with other characters such 

 as those of the hinge (see p. 218) and the ribbing of the shell, it 

 becomes useful as a means of diagnosis. 



From the observations above recorded we can now distinguish more 

 clearly the several groups into which the genera Mytilus and Modiola 

 may be divided, and shall be able to form some idea of their generic 

 or subgeneric value. 



Genus Mytilus, Linngeus. 



TJmbones generally subterminal, but sometimes terminal ; anterior 

 expansion of the shell veiy small or obsolete. Hinge with a variable 

 number of small teeth on the anterior side, these teeth being connected 

 with an equal number of small short riblets which curve inwards 

 from beneath the umbones. Outer sui-face of shell smooth, except 

 in the subgenus Aulacomya. Anterior muscular scar small, and in 

 some cases absent. 



Subgenera. 



Eumytilus, Ihering. Anterior side with a small ribbed expansion 

 under the umbo, the margin of which bears several small teeth, 

 generally from 3 to 5, but sometimes only 2 ; these teeth vary in 

 size and number, even in the same species. Anterior adductor scar 

 always present, the median and posterior byssal scar united to one 

 another and to that of the posterior adductor. Type, M. edulis, Linn. 



To this group belong the following species : — 



Op. cit , pp. 85, 87. 



" Catalogus Conchyliorum quae reliquit Comes de Toldi, Hafaiae," 1853, pt. ii 



p. 52. 

 Mr. Purdie seems to have found that an anterior adductor was always present in 



the New Zealand form of M. 3Iagella>iicHs. I have not been able to ascertain 



whether it differs in any other respects from that species. 



