90 monograph: of the genus gnathodon-dall. 



Clathradon (Gray MS.), Conkad, Am. Journ. Sci., xxiii, p. 340, Jan., 1833. 



Lapsus for Clathrodon, Gray MS., 1830. 

 Clathoilov, SowERBY, Man., 2d etl., p. 108, 1842. 

 Ferissodon, Conrad, Proc. Acad. Nat. Sci., Phila., 1862, p. 573, 1863 (Type F. 



Grayi, CoyR\D=Maetra clathrodonta, Coxrad, 1833). 

 liangianella, Conrad, Am. Journ. Conch, in, suppl., p. 30, 1867 (Type G. 



trigonnm, Petit, Mazatlan, Mexico). 



Shell trigoiial, equi valve, closino- completely ; iirabones proniineut, not 

 adjacent, smooth at the point of origin, erector twisted forward; lunule 

 and escutcheon obscure or absent; shell-substance porcelain-white 

 internally; externally chalky, with a thin epidermis; anterior shorter 

 than the posterior end; the latter produced or rostrate; hinge compris- 

 ing a bifid triangular cardinal tooth in one valve over which fit two 

 lamellar divergent teeth of the opposite valve, an accessory lamella 

 sometimes rising from the anterior edge of the cartilage pit next the 

 cardinals; an anterior lateral tooth in one valve received between two 

 less prominent lamiuie of the opposite valve, of which pair the dorsal 

 lamina approaches nearer the cardinal tooth than the ventral one, 

 leaving a gap into which the proximal end of the anterior lateral, when 

 adult, is more or less distinctly hooked; a longer jiosterior lateral in 

 the same valve as the anterior tooth, received between two subequal 

 less prominent laminae in the opposite valve; teeth crenulated or 

 granulose on their opposed surfaces; cartilage pit deep, persistent; 

 internal border of the valves smooth or faintly radiately striated; 

 adductor scars distinct, the anterior smaller; pallial line distinct, dis- 

 tant from the margin ; pallial sinus small, rather irregular; cartilage 

 large, inserted on the ventral surface of the pit, persistent in its entirety, 

 so that its distal ends sometimes ])roject from the eroded umbones; 

 ligament wholly internal, small, inserted on the dorso-posterior surface 

 of the pit and separated by a shelly ridge on each side from the cartilage 

 below it; mantle-edge smooth, simple, the lol^es marginated, the inner 

 edge of the margin thicker and elevated, the lobes free edged from 

 below tlie anterior adductor nearly to the siphons ; antesiphonal chan- 

 nel of the incurreut siphon longitudinally divided bj^ an elevated raphe 

 arising from the inner surface of the mantle; siphons moderate, united 

 to their tips, their distal orifices sparsely papillose; the proximal orifice 

 of the incurreut siphon with an imperfect arched valve; gills two on 

 each side, the inner larger, suspended by its base; the outer smaller, 

 its line of attachment crossing the gill obliquely and forming of the 

 upper portion an "appendix" which is soldered to the mantle by most 

 of its dorsal surface; all four gills united behind the foot, their proxi- 

 mal portion forming a septum which is anchored to the anterior portion 

 of the siphonal sejitum, thus completely separating the anal and peri- 

 pedal chambers ; palpi, four in number, narrow, long, internally striated, 

 externally smooth, the lower pair continuous medially in front of the 

 foot; foot small, compressed, short, angular in front, pointed behind, 

 ventral edge sharp, entire ; byssus and byssal gland atrophied or absent 

 in the adult. 



