360 MOLLUSCA SUB-KINGDOM VI 
There are no forms with a filibranchiate gill, or with a typically fully developed 
reticulate gill. 
‘ 
Teleodesmacea.—Pelecypods with reticulate gills, the ventricle of the heart embracing 
the rectum ; having the mantle lobes more or less connected and usually possessing developed 
siphons ; the adductors practically equal ; the shell structure cellulo-crystalline (porcellanous) 
or obscurely prismatic, never nacreous ; the dorsal area, when-preseut, always prosodetic or 
divided into lunule and escutcheon ; ligament opisthodetic, with or without separate resilium ; 
without a lithodesma, rarely with external accessory shelly pieces ; nepionic stage usually 
without a Taxodont provinculum ; permanent armature of the hinge characterised by the 
separation of the hinge teeth into distinct cardinals and laterals; the posterior laterals, 
when present, placed behind the ligament ; the animals active or nestling, sometimes sessile, 
but rarely sedentary burrowers, rarely inequivalve, usually possessing a hinge plate and a 
pallial sinus ; sexes usually separate. 
It is doubtful if this group is represented in the Palaeozoic rocks, especially below 
the Carboniferous, though genera belonging to it are foreshadowed by some of the 
Palaeoconchs. Although most of the Teleodonts live embedded in the surface of the 
sea-bottom, they retain their ability to migrate, and only a few extremely specialised 
forms inhabit permanent burrows of their own construction. They are sometimes 
commensal in the burrows of other animals. Similarly, few of them fix themselves 
permanently by a byssus, although often byssiferous, especially when young. With 
the exception of a few specialised forms they possess a pair of direct and reflected 
branchial laminae on each side of the body, frequently united behind the foot, forming 
an anal chamber; the two sets on one side usually of unequal size, and of the reticulate 
type. None are known with typically foliobranch or filibranch gills, although some 
abyssal forms have archaic sub-foliobranchiate ctenidia. 
There remains a small group of fossils, difficult to refer to a place in the system, 
yet characterised by several features in common ; these have been named by Neumayr 
Palaeoconcha, and are defined by him as follows :— 
Palaeoconcha.—Prototypic Pelecypods, with thin shells, a simple or obscure pallial line, 
sub-equal adductor scars placed high in the valves ; dorsal area absent or amphidetic ; liga- 
ment external, variable ; hinge margin edentulous or with polymorphous teeth formed by 
modifications of the margin and not set upon a hinge plate. 
While the forms included here are not always actually the most ancient, yet in 
their modifications they indicate clearly the origin of many subsequently developed 
structures found in Pelecypods of a more modern type ; and owing to their undifferen- 
tiated polymorphic character are difficult to assign a place in any classification based 
on more highly developed forms. There is little doubt that some of these show Taxo- 
dont affinities, and others recall Pholadomya ; but the final discussion of these puzzling 
forms awaits greater knowledge of them and other early bivalves. It is to be under- 
stood that the places assigned them in the present systematic arrangement must be 
more or less provisional. Neumayr included in this group the following families :— 
Viastidae. ' Praecardiidae. Solenopsidae. 
OCardiolidae. Stlurinidae. Grammystidae. 
Antipleuridae. Protomyidae (including possibly Posidonomyidae. 
Lunulicardiidae. the recent Solemya). Daonellidae. 
The pelagie Planktomya Henseni, recently described by Simroth, presents many of 
the characteristics of the: Palaeoconchs. The posterior cardinal margin is denticulate, 
the ligament internal, and the gills are represented by a single lateral plate parallel 
with the longer axis of the shell on each side ; a type elsewhere only known in con- 
nection with thé younger stages of Scioberetia. 
