CLASS I PELECYPODA 359 
made parallel to or induced by the presence of the chondrophore and 
resilium. 
There is some reason to think that the presence of the resilium in Pecten 
and Spondylus may be connected with those changes of the auricular 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 or- 
dinal groups: Prionodesmacea, 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 when so divided, having an amphidetic ligament ; ligament variable, rarely 
opisthodetic ; nepionic stage usually with a taxodont provinculum ; 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 archaic 
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 completely 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 amphi- 
detic or obscure, rarely distinctly divided; the ligament usually opisthodetic, generally 
associated with a separate resilium, chondrophores, and lithodesma ; valves generally unequal, 
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 Pulaeoconcha, 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 approximates the 
Myacean Teleodesmacea, It is peculiar in the possession of a lithodesma, 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 
foliobranch gill have it different from the foliobranch gill of Prionodesmacean groups. 
