ORDER III TELEODESMACEA 493 
Family 44. Gastrochaenidae. Gray. 
Shell substance as in Saxicava ; valves equal, widely gaping in front ; adductor scars 
unequal, the anterior smaller ; pallial sinus deep, margins simple; area none; ligament 
and resilium external, parivincular ; hinge with a single obsolete cardinal or wholly 
edentulous; animal frequently forming 
an external protective tube to supplement 
its burrow, but to which it is in no way 
attached. ? Permian. Trias to Recent. 
This group stands between the Myacea 
and <Adesmacea, verging on the latter. 
Many of its characters are adaptive, and are 
FiG. 783. repeated in the Hnasiphonacea, but morpho- 
Gastrochaena Deslong. logically its relations to the Saxicavidac 
champsii, Laube. Middle seem close. 

Gastrochaenaangusta, Desh. Jura; Balin, near Cracow. 
Eocene (Sables moyens) ; Cast of burrow including , 4 a Ta) tis 
Valmandois, near Paris. one of the valves, 1/). Gastrochaena, Spengler (Chaena, Retz. ; 
Rocellaria, Blainv.), (Figs. 782, 783), 
Bores cylindrical or pear-shaped cavities im rock, shell, or coral. Trias to Recent. 
Fistulana, Brug. Secretes calcareous tubes which stand upright in the sand or mud, 
Superfamily 17. ADESMACEA. Blainville. 
Gills with direct and reflected laminae, long, united, extended into the branchial 
siphon ; posterior adductor usually in front of the visceral ganglion, anterior adductor 
external to the cavity of the valves, exerted in a contrary sense to the posterior muscle ; 
hinge margin reflected, edentulous; ligament obsolete; a myophoric process extending 
freely into the valve from the sub-umbonal cavity. 
Family 45. Pholadidae. Fischer. 
Shell cellulo-crystalline, with a thin epidermis ; valves more or less gaping in front 
and behind, with inconspicuous beaks and reticulate, often spinose sculpture; in the 
adult supplemented by accessory shelly pieces, always attached to the valves, but not by 
an exterior shelly tube like that of the Gastrochwenidae ; the antero-dorsal margins more 
or less extensively reflected, the postero-ventral approximated ; pallial line sinuated, area 
none; ligament and resilium usually absent, an obsolete remnant of the resilium and 
chondrophore sometimes present in the left valve. ? Carboniferous, Jura to Recent. 
Pholas, Linn. (Fig. 784). Surface divided by grooves into areas which often have 
diverse sculpture ; the adult often provided with accessory shelly plates, each of which 

FIG. 754. 
Pholas Levesquei, Watelet. Eocene; Cuise de la Mothe. 
when seated in front of the beaks has heen named a “ protoplax,’ when above the 
beaks, “mesoplax,” when behind the beaks between the valves “ metaplax,” and when 
between the valves ventrally (Martesia), “lypoplax.” A calcareous septum, secreted 
after the completion of the burrow, and occupying the pedal gape of the valves, is 
called the “ecallum.” The addition of these plates and appendages during growth so 
