222 
inent rounded edge, is marked with numerous very close- 
set vertical strie, and does not extend so far in front as. 
behind, where it gradually narrows and yanishes. It is. 
divided by a rather wide, shallow, subcentral triangular carti- 
lage-pit. There are four anterior teeth perpendicular to the. 
margin, and four posterior convexly curved towards the 
umbo; three or four small marginal teeth inside the anterior 
end, four at the post-dorsal margin, and three or four obso- 
lete teeth at the post-ventral border. Margin otherwise: 
simple. Posterior muscle-scar long, large, triangular. Crowd- 
ed concentric, rather irregular accremental strie, with very 
fine broken radial incisions. 
Dimensions.—Antero-posterior, 2.4 mm.; umbo-ventral,. 
2 mm. 
Locality.—MacDonnell Bay and Guichen Bay, in shell- 
sand. 
Variations.—When dead they become white, the amber: 
colour disappearing first about the umbo, and last about the- 
ventral margin. Some show four obscure rounded radial 
ridges, from the umbo to the posterior inferior angle; and 
these may crenulate the margin. Dorsally to these the shell 
may be somewhat hollowed. The posterior marginal teeth 
may crenulate the border, as may also the front teeth. 
AMiviociutullagathyarca perversidens, Hedley. ~ 7 Ss 
Bathyarca perversidens, Hedley, Memoirs of Australian Mus.,. 
1902, vol. iv., part 5, p, 298, fig. 45. ype locality, off Port 
Kembla, in 63-75 fathoms, ete. Var.—Bathyarca cybea, Hed- 
ley, Trans. New Zealand Inst., 1905, vol. xxxviii.. 1905, p. 70, 
pl. 1, figs. 8-4. Type locality, 110 fathoms, east of Great Barrier: 
Island, New Zealand. 
Dredged off Cape Jaffa, in 130 fathoms, 2 valves; in 
300 fathoms, very many valves. 
A series was submitted to Mr. Hedley as B. cybaa, and 
he kindly compared with his types and confirmed the identi- 
fication. A suggestion was also put forward that his two 
shells were variants of one species, and he allowed that my 
series seemed to bridge the interval between the two. Being: 
persuaded they are conspecific, I have named them B. per- 
versidens, which has priority, making 2B. cybea a variant. 
In all my specimens the right valves have much more. 
pronounced sculpture than the left, especially the radial. 
Some shells have the sinuation at the gape quite deep, with 
the depression from this to the umbo marked, others but 
slight, and others not at all. Some have quite a distinct. 
angle at middle of the posterior side, where a straight-dor- 
sal half meets the rounded ventral part; in others the pos- 
terior side is continuously curved. 

