85 181 
Montacuta costata n. sp. 
(Pl. II, Fig. 27). 
This little species is oblong-ovate and rather flat, white, semilransparent, some- 
what glossy and provided with a thin yellowish epidermis. The anterior end is 
regularly rounded, the posterior end forms a somewhat sharp angle with the dorsal 
side, and then slopes in a straight line down towards the ventral side, with which 
it forms a roundly-acute angle; a sharp keel extends from the latter across the 
shell up towards the apex. The ventral margin is rather straight. The umbones 
which are not very prominent, are situated towards the posterior end. The sur- 
face is smooth around the apex, but gradually slight, concentric cosle are seen to 
appear, which upon the rest of the shell (about 1 mm. from the top right down 
to the ventral margin) occur as numerous, close-set, parallel, uniform, rounded 
ridges; at the keel mentioned above they turn abruptly and ascend parallel to each 
other straight up towards the sloping posterior side of the upper margin. The 
interior of the valves is white and glossy. The left valve has two erect lamellar 
very divergent long teeth, of which the posterior is rather more strongly developed; 
they are separated by a broad triangular space, and stand out freely from the outer 
margin. The shell-margin is somewhat thickened, especially along the ventral side. 
Long. 3 mm., alt. 1:7 mm., crass. 1 mm. 
West of Koh Kut, 30 fathoms, sand and mud (1/2). 
Unfortunately only a single left valve of this little species was obtained, and 
it is perhaps not full-grown; when complete specimens are at hand, it will be 
proved whether I am right in referring it to the genus Montacuta. With regard to 
the generic name Montacuta, it was given in 1819 to this group of the Lamelli- 
branchiata by Turron, under the form of Montacuta (after the zoologist GEORGE 
MontaGu). In 1828 Fremina employed the generic name Montagua for a group of 
Aeolidae, but Rup. Bercu afterwards (1864) replaced this by the name Cratena; in 
1825 DesMAREsT employed the name Montagua for a crustacean, but this was after- 
wards obliged to give place to Callianassa, Leach. In 1852 Lreacn proposed the 
name Montagua for a group of Trochus-forms (= Jujubinus, Monterosato). BRronn, 
and later Bucguoy, DaurzENBERG and Do.irus, and also Lamy, use the name 
Montaguia for this group of Lamellibranchiata,' but according to the rules for 
zoological nomenclature now in force this is not admissible, it should be Montagua, 
if it could be used at all. 
Montacuta venusta n. sp. 
(Pl. III, Figs. 28—29). 
This species is oblong-ovate, donaciform, rather flat and white. The umbones 
are siluated towards the front end, in the anterior third of the shell. The anterior 
end slopes in a slight curve abruptly down from the apex towards the ventral 
1 P, FiscHER says also (Manuel de Conchyliologie, p. 1027) that Montaguia is the most correct 
form of the name. 
