208 PROCEEDINGS OF THE MA.LACOLOGICAL SOCIETY. 



the superior cardiual (always in the right valve, normally) I call the 

 *' arms " of the cardinal tooth. The sinole tooth of the left valve has 

 sometimes developed corresponding arms, but otherwise may be thick, 

 triangular, grooved below in the middle, and turned up like the petal 

 of a lily. This latter condition I call "petaloid." It is conspicuous 

 in some forms of Spisula. In some ]\Iactroids with deeply excavated 

 hinge -plate the sinus is roofed over, and a bristle may be inserted into 

 a deep cavity nearly to the beaks. While the dorsal lamina is set on 

 the valve, the ventral lamina ai-ises from the plate ; both are elegantly 

 arched and somewhat twisted. This may be well observed in Ccclo- 

 mactra violacea. 



Having endeavoured to give some idea of the diversity of character 

 to be found in the hinge-apparatus of Ifactra, which can only be 

 adequately studied specimens in hand, it remains to observe that in a 

 very complete series of fossil forms, taken with the recent ones, it will 

 be found that the various types closely approach one another by the 

 agency of border species. This is what we find in all groups of 

 animals Avhen sufficiently full series are studied, and what, by the 

 theory of evolution, we ought to expect. In all large groups it 

 becomes necessary to name what may be called, with reference to the 

 group, the successive stages, in order that they may be adequately 

 recognized, and this is what, in the accompanying schedule, I have 

 attempted. It is only just to acknowledge that a long step was taken 

 in this direction so far back as 1853, by Dr. J. E. Gray, and, save for 

 some errors of identification, the system of to-day was quite fully 

 indicated in its principal features in his classification. 



The following characters are those which I find in the different 

 larger groups under consideration ; though, of course, there are 

 numerous cases where it has been impracticable for me to obtain 

 the soft parts of species, which I have consequently been obliged to 

 classify by the shell. 



Family M A C T R I D iE . ^ 



Siphons united to their tips. 



Subfamily MACTRIN^. 



Shell subequilateral, nearly closed; hinge normal (as above de- 

 scribed), fully developed ; siphons partially or wholly naked, and 

 wholly retractile within the shell; mantle lobes separated ventrally 

 between the siphons and the anterior adductor. 



Subfamily PTEROPSIDINiE. 



Shell subequilateral, nearly closed ; hinge feeble, concentrated ; the 

 laterals much reduced or partly obsolete ; siphons wholly retractile, 

 naked ; mantle partially closed ventrally. 



Only (liffereutial characters are mcutioued uudur the family names. 



