436 MOLLUSCA phylum vi 



There is some reason to think that the presence of the resilium in Peden 

 and Sjpondylus may be connected with those changes of the anricular crura 

 which lead to the assumption of dental functions by the latter. But it is well 

 known that submergence of the resilium occurs independently in many unre- 

 lated groups of bivalves ; and it is probable most of them were previously 

 dentiferous and still retain their teeth, although more or less modified or 

 displaced, while the edentulous genera seldom show any teeth which appear 

 to owe their existence solely to the presence of a chondrophore. The nearest 

 approach to a hinge composed of dental laminae of such an origin is found in 

 Placuna, Placenta and Placunanomia, together with the Spondylidae already 

 mentioned. 



Classification. — The class Pelecypoda, which comprises about 5000 recent 

 and twice as many fossil species, appears to be divisible into three ordinal 

 groups : Prionoclesmacea, Anomalodesmacea and Teleodesmacea ; of which the 

 third represents the most perfected and developed (though not always the 

 most specialised) modern type of bivalve. There seems little reason to doubt 

 that all these orders are descended from a Prionodesmatic radical or prototype, 

 and that for various reasons the first and second retain more evident traces of 

 this origin than the third. 



For convenience of comparison, the characteristics of these orders will be 

 stated here. 



Prionodesmacea. — Pelecypods having the lobes of the mantle generally separated, or, 

 when caught together, with imperfectly developed siphons ; the soft parts in general 

 diversely specialised for particular environments ; the shell structure nacreous and prismatic, 

 rarely porcellanous ; the dorsal area amphidetic or obscure, rarely divided into lunule and 

 escutcheon, and wlien so divided, having an amphidetic ligament ; ligament variable, rarely 

 opisthodetic ; nepionic stage usually with a taxodont provinculura ; permanent armature of 

 the hinge characterised by a repetition of similar teeth upon the hinge line, or by amorphous 

 schizodont dentition ; habits active, sessile or nestling, not burrowing ; monoecious or 

 dioecious. 



This group, originating with the earliest forms, has retained many arcliaic 

 features through immense periods of geological time, although occasionally developing 

 remarkable and persistent specialisations. Notwithstanding most of its subdivisions 

 have arrived at a notable degree of distinctiveness, intermediate forms of ancient date 

 connect them all, more or less effectively, with the parent stem. 



Anomalodesmacea. — Pelecypods having the mantle lobes more or less completelj' united, 

 leaving two siphonal, a pedal, and sometimes a fourth opening between them ; siphons well 

 developed, always at the posterior end of the body ; two subequal adductor muscles ; the 

 shell structure nacreous and cellulo- crystalline, rarely with a prismatic layer; the area 

 amphidetic or obscure, rarely distinctly divided ; the ligament usually opisthodetic, generally 

 associated with a separate resilium, chondrophores and lithodesma ; valves generally un- 

 equal, the dorsal margin without a distinct hinge plate, armature of the hinge feeble, often 

 obsolete or absent ; rarely with lateral laminae or well-developed dental processes ; usually 

 burrowing, hermaphrodite, and marine. 



This group is intimately related to many of the Palaeoconcha, except as regards 

 the presence of a pallial sinus. • It retains many archaic features, and includes several 

 of the most specialised modern forms. Through the Anatinacea it aj^proxiuiates the 

 Myacean Teleodesmacea. It is peculiar in the possession of a litliodesma, and in the 

 structure of its gills and hinge. The forms with a reticulate gill have it of a different 

 type from the reticulate gills of the other orders ; those which retain a modified 

 foliobraiu'h gill have it diffei'cnt from the fbliobrancli gill of Prion odesmacean groups. 

 There ai'e no forms with a filibranchiate gill, or with a typically fully develoj^ed 

 reticulate gill. 



