ORDER I PRIONODESMACEA 363 
type of a distinct family, may also be included. It is distinguished by its feebler 
structure and a dorsal radial groove near the cardinal border. 
II, Taxoponta. Neumayr (emend.) 
Superfamily 1. NUCULACEA. 
Shell of variable form, closed ventrally, equivalve, with a smooth epidermis ; nacreous or 
porcellanous with tubuliferous external prismatic layer ; area obscure, or none, when present 
divided into lunule and escutcheon ; ligament variable, amphidetic ; gills 
foliobranchiate ; both adductors present and sub-equal ; foot grooved and 
reptary, not byssiferous ; marine. 
Family 8? Ctenodontidae, Dall. 

Shell Nuculiform, with-the teeth in a continuous arched series ; no 
area ; ligament external, alivincular, without an external resiliwm ; 
- . c é . . os 1: . Ctenodonta pectun- 
pallial line simple. Ordovician and Silurian. culoides, Hall, OF: 
: , : rape ! . . . dovician ; Cincin- 
Ctenodonta, Salter (Tellinomya, Hall P-P-), (Fig. 603). Oval, smooth jy ati Ohio. 2h (after 
or concentrically striate, in the later horizons sometimes Ledaeform, Hall). 
Ordovician and Silurian. 
- Cucullella (M‘Coy), Fischer. Ovate, thin-shelled, with a straighter hinge line and 
a radial buttress to the anterior adductor. Silurian. 
Fia. 603. 
Family 9. Nuculidae. Adams.! 
Shell compact, closed, with the teeth in two series meeting below the wumbones, separated 
by « chondrophore ; area represented by an obscure 
lunule and escutcheon; no ligament, but a wholly 
internal, amphidetic, alivincular resilium ; internal 
layer of shell nacreous; mantle lobes free, without 
siphons ; pallial line simple. Ordovician to Recent. 
Nucula, Lam. (Nuculana, Link), (Fig. 604). 
Oval or triangular, concentrically or reticulately 
sculptured. Silurian to Recent. Represented by 

Fra. 604. over 200 fossil and half as many recent species. 
A: Nucula strigilata, Goldf. Upper Trias ; Acila, Adams. With divaricate sculpture. 
St. Cassian, Tyrol. 1/4. B, N. nucleus, Linn. { 
Miocene ; Grussbach, near Vienna. 1/}. Lower Cretaceous to Recent. 
Family 10. Ledidae. Adams.! 
Shell as in the Nuculidae, but elongated with the ligament variable, the resilium sometimes 
external or absent, the internal shell layer sub-nacreous or 
porcellanous, the ends of the shell partly gaping ; the mantle 
lobes more or less wnited ; with complete, sometimes elongate 
siphons ; pallial line usually sinwated. Silurian to Recent. 
Cleidophorus, Hall (Adranaria, Mun. Chalm.), (Fig. 
605). Shell rostrate, the anterior side shorter, with an 
internal radial buttress. Silurian and Devonian. Fic. 605. 
Cytherodon, Hall, Silurian and Devonian. ? Redonia, _ Cleidophorus cultratus, Sandb. 
Ronault ; Cadomia, Tromelin; Tellinomya, Palaeoneilo, PN Te cateahuetein hence re 
Hall ; Anuscula, Barr, ; and Myoplusia, Neumayr. Silurian. 
Leda, Schum. (Figs. 606, 607). Shell rostrate, elongate, often keeled, concentri- 
cally striate ; hinge as in Nucula; pallial sinus small. Silurian to Recent. 
' Verrill, A. E.,and Bush, K. J., Revision of the genera of Ledidae and Nuculidae (Amer. Journ. 
Science [4], vol. III. pp. 51-63), 1897. 

