8'.4 PELECYPODA. [Eulamellibranchia. 



Fam. MODIOLARCID^E, Gray. 



Mantle with 2 sutures ; the foot byssiferous, with a plantar surface 

 and a glandular cavity in front of the byssogenous cavity ; the two 

 branchial plates serve as incubatory pouches. 



Shell equivalve, covered with a hard polished epidermis. Hinge- 

 teeth none or rudimentary ; ligament linear, external. 



Living attached to floating seaweeds. 



Genus 1. MODIOLARCA, Gray, 1847. 



Modiolarca, Gray, P.Z.S., 1847, 199. Type : J/. trnjjt~ina, Lam. Gai- 

 mardia, Gould, 1852. Phaseolicama, Valenciennes, 1854. 



Animal having the mantle united, with the exception of 3 openings 

 a small one for the foot, one branchial, and one anal ; branchiae un- 

 equal, the outer having only two-thirds the size of the inner ; foot 

 linguiform, grooved, byssiferous. 



Shell ovate, trapezoidal, thin, fragile, ventricose ; surface of valves 

 covered with a smooth shining epidermis ; beaks anterior, prominent, 

 contiguous ; anterior end compressed, sloping abruptly to the base ; 

 hind margin rounded, ventral margin somewhat sinuous and gaping 

 anteriorly. Hinge with 2 small oblique teeth in the right valve, which 

 receive 2 corresponding ones in the left. Muscular impressions distinct. 



The genus is subantarctic. 



KEY TO SPECIES. 



A. Shell rostrate at the lower anterior end. 



a. Rostrum narrow, distinct ; shell small, maximum length 



about 5 mm. 



b. Rostrum very short, acute ; beaks near the anterior end acrobeles. 

 bb. Rostrum long and narrow ; beaks at about the anterior 



third . . . . . . . . . . tasmanica. 



aa. Rostrum broad, sometimes inconspicuous ; maximum length 



of shell about 17 mm. . . . . . . . . trapezina. 



B. Shell oblong, not rostrate. 



a. Shell oblong, beaks a little in front of the middle ; white and 



reddish .. .. .. .. .. .. Smith/. 



aa. Shell subcylindrical, beaks nearly terminal . . . . pusilla. 



1. Modiolarca acrobeles, n. sp. Plate 52, fig. 5. 



Shell very small, thin, transversely elongate, very inequilateral, 

 the beaks nearly anterior, shortly rostrate in front, moderately ventri- 

 cose, with a few indistinct radial lines, white. Beaks rounded, incurved, 

 slightly turned forward, very near the anterior end. Anterior end 



O " J * 



very short, compressed, shortly but rather acutely rostrate below, 

 the upper margin subvertical, excavated ; posterior end long and broad, 

 convex, the dorsal margin straight ; ventral margin sinuated in front, 

 descending and broadly rounded behind. Sculpture consisting of con- 

 centric growth-lines, and occasionally a few radial lines at the median 

 part of the valves. Colour white, but fresh shells are very likely 

 brown. Interior white (dead shells), smooth. Margins sharp, smooth. 



