222 PROCEEDINGS OF THE MALACOLOGICAL SOCIETr, 



for the o of ocoutc's cannot be omitted, and if emendation of any kind 

 is permitted, the name must be converted into Brachyodontes. Per- 

 sonally I should like to replace it by Brachyodon^ which would more 

 correctly convey the meaning of a 'short-toothed shell,' but as the 

 plural form has been allowed to stand in Macrodontes and Polydontes 

 (properly Polyodontes) I do not alter it in this case. 



There is another small group of mytiloid shells which differ very 

 little from those which form the subgenus Morinomya, but has been 

 regarded by some authors as having generic rank. This is the Septifer 

 group, in which the anterior adductor muscle is attached to a myophoric 

 plate instead of to the inner surface of the shell. In all other respects, 

 so far as I can learn, both shell and animal agree with Hormomya, 

 and the existence of a myophoric septum does not seem of sufficient 

 structural importance to form the basis of a separate genus. 



It is true that most species of Septifer are stout, strongly ribbed 

 shells, but in these respects they resemble B. {S.) hirsutiis and 

 B. purpiiratus ; they also compare with these species in the length 

 and strength of the ligament, and in the small number of posterior 

 crenulations on the hinge-line, which have been left unobliterated 

 by its extension. It seems natural, therefore, to place Septifer as 

 a subgenus under Brachyodontes. All the species with a crenulated 

 hinge-line will come under these four groups except M. hamattis (Say), 

 which is exceptional and interesting in several particulars. In the 

 first place, the ligament is very long, is completely marginal, and 

 seems to have almost obliterated all the posterior crenulations of the 

 hinge-line, for, though the shell-margin is crenulated by the ribs, 

 these crenulations become oblique and faint near the ligament, instead 

 of becoming stronger and transverse. It is a fact, however, that this is 

 sometimes the case with that variable species Brack, ptvpuratus, Lamk., 

 and its var. ovalis. M. hamatus, however, has other peculiarities : 

 there is no anterior adductor, and the united posterior byssal and 

 adductor muscles leave a scar of unusual shape, the adductor end of 

 which is little broader than the byssal end. 



As all these peculiarities are not combined in any other species 

 with which I am acquainted, I consider that M. hamatus stands by 

 itself, and should form a separate subgenus of Brachyodontes, for which 

 I propose the name of Ischadiuni, thinking that in shape and colour 

 the shell has a sufficient resemblance to a ' di'iod fig' to justify the 

 appellation. 



The genus and subgenera can then be defined as follows : — 



Genus Brachyodontes, Swainson. 



Shell generally finely ribbed, but sometimes wrinkled, or even 

 smooth. Anterior development of the shell, and consequently the 

 position of the umbones, variable. Hinge-line generally straight, 

 and terminating in an angle posteriorly. Ligament rather short, 

 marginal or infra-marginal. Anterior margin with several close-set 

 teeth ; posterior border crenulated, sometimes throughout its length, 

 above and behind the ligament, sometimes only behind the ligament. 



