CLASS I PELECYPODA 437 



Teleodesmacea. — Pelecypods with reticulate gills, the ventricle of the heart embracing 

 the r^'ctiun ; having the mantle lobes more or less connected and usually possessing developed 

 siphons ; the adductors practically ecjual ; the shell structure cellulo-crystalline(porce]lanous) 

 or obscurely prismatic, never nacreous ; the dorsal area, when present, 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 ; pei'manent armatm-e 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 Paleozoic rocks, especially below 

 the Carboniferous, though genera belonging to it are foreshadowed by some of the 

 Palaeoconclis. 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 ow^n construction. They are sometimes 

 commensal in the burrows of other animals. Similarly, few of them fix themselves 

 Ijermanently by a byssus, although often byssifei'ous, 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 

 tyj^e. None are known with typically foliobranch or filibrancli 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, Mith thin shells, a simple or obscure pallial line, 

 sub-equal adductor scars placed high in the valves ; dorsal area absent or amphidetic ; liga- 

 ment externa], vaiiable ; 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 tlie most ancient, yet in 

 their modifications they indicate clearly the origin of many subsequently developed 

 structures found in Pelecyj)ods 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 ; — 



Vlastidae. Fraeeardiidae, Solenopsidae. 



Gardiolidac. Silurinidae. Grammysiidac. 



AntipleiD-'idae. Protomyidac (including possibly Posidonoviyidae. 



Lunulicardiidac. the Recent Solemya). Daoncllidae. 



The pelagic 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 rejjresented by a single lateral plate parallel 

 with the longer axis of the shell on each side ; a type elsewhere only knowar in con- 

 nection with the younger stages of Scioheretia. 



