28 
PACIFIC SCIENCE, Vol. XXI, January 1967 
Fig. 19. Quadracythere hornibrooki n. sp. a—c, Holotype usnm 648758; a, lateral view of left valve; 
b, internal view; adductor scars in white; c, dorsal view, d, Paratype usnm 648760; muscle scar pattern 
of right valve, e, Paratype usnm 648761; muscle scar pattern of left valve; adductor scars in white. 
oped narrow ridge from eye tubercle along dor- 
sum turning down in posterodorsal area forming 
sinuous median ridge terminating at subtle sub- 
central tubercle; prominent ventral ridge pos- 
teriorly extended forming weak alae; surface 
with large, deep various-sized reticulations. 
Dentition holamphidont: median element of 
left valve a smooth bar with stubby anterior 
tooth, bar thickening posteriorly; posterior 
tooth of right valve entire, reniform; posterior 
socket of left valve with small ventral and 
posterior teeth. 
Adductor muscle scar pattern complex; top 
scar entire, second scar down divided into two 
or three scars, bottom two scars variously divided 
into 3-5 smaller scars. Two antennal scars 
located anterior to upper adductor scars, large 
dorsal mandibular scar present. Normal pores 
of sieve type, large, forming deep pits internally. 
dimensions: See Table 6. 
distribution: As fossils from T-2 (1 valve), 
T-ll (1 valve), and T-12 (26 valves; 2 entire). 
discussion: Generic discrepancies in denti- 
tion and musculature exist between species of 
typical Quadracythere from New Zealand and 
the present species. In the latter, the median 
bar is smooth, not crenulate, and the adductor 
scar pattern is usually divided. 
The Hawaiian individuals are somewhat com- 
parable to Quadracythere mediaruga Horni- 
brook (Kaiatan-Recent, New Zealand) in shape 
and placement of ridges, but lack the antero- 
