— oe a 
- der ogsaa er bemerket af Grube. 
119 
at man faar se den lige forfra (Fig. 7), viser den 
sig at have sin storste Brede nermere Dorsalsiden, 
medens Valvlerne nedad stoder sammen under en 
temmelig spids Vinkel. Den dorsale Fure viser sig 
i denne Stilling af Skallen som en dyb Indbugtning 
oventil. I den forreste Del af hver Valvel sees 
mere eller mindre tydeligt den rundagtige Inser- 
tionsarea for Skallens Lukkemuskel og umiddelbart 
bag denne Skalkjertelen. Denne sidste (se Tab. 
XIX, Fig. 16) viser i alt veesentlig samme Bygning 
som hos Limnadia, men er forholdsvis kortere og 
bredere. 
Skallens Overflade er ganske glat, uden Spor 
af Vextstriber. Rigtignok sees under visse Belys- 
ninger, i nogen Afstand fra Valvlernes frie Kanter, 
ligesom en svag buet Linie (se Tab. XVIII, Fig. 6), 
men jeg har overbeyist mig om, at dette ikke er 
nogen virkelig Vextstribe, men skriver sig fra den 
Omstendighed, at Valvlernes Randparti viser en 
noget anden Structur end den ovrige Del, noget 
Skallen bestaar 
forovrigt, som hos Limnadia, af 2 Lameller, en ydre 
temmelig sterk, men meget elastisk chitinos La- 
melle, og en seerdeles delikat indre Membran, der 
danner en Fortsettelse af Legemets Integument. 
Ved meget sterk Forstorrelse viser Skallens ydre 
Lamelle regelmessige rundagtige Masker eller For- 
dybninger. Kanterne af Valvlerne er noget fortyk- 
kede og fuldkommen glatte. 
Selve Legemet er (se Tab. XIX, Fig. 1) feestet 
til Skallen paa en fuldkommen lignende Maade som 
hos Limnadia, nemlig ved et smalt dorsalt Ligament, 
der udgaar indad fra Skallens mest fremspringende 
Del, og i nogen Afstand under dette ved Adductor- 
muskelen. Legemet er paa denne Maade ophengt 
paa en lignende Maade som hos hin Slegt i Skal- 
lens Hule, saaledes at baade den forreste og bager- 
ste Del er frit bevegelige. I Forhold til Skallen 
er imidlertid Legemet hos neerverende Form meget 
mere volumingst end hos Limnadia, hvorfor ogsaa i 
Regelen, saavel hos levende som Spiritus-Exemplarer, 
en storre Del af Hovedet rager frem af samme for- 
til (se ogsaa Tab. XVIII, Fig. 1 og 2). Ved en 
meget sterk Boining af Hovedet, hvorved dette 
legges ind mod Bugfladen af Kroppen, kan dog 
dette, ligesom det ovrige Legeme, helt inddrages 
indenfor Valvlerne, som da slutter tet sammen 
overalt. Som hos Limnadia, falder Legemet natur- 
ligt i 2 Hovedafsnit, et forreste og et bagerste, 
begreendsede fra hinanden ved det dorsale Ligament 
og Skallens Lukkemuskel; men Forholdet mellem 
disse 2 Afsnit er her et helt andet, idet det forre- 
ste er fuldkommen ligesaa stort som det bagerste. 
Begge Afsnit lader sig igjen dele i 2, det forreste 
i Hovedet og Nakkesegmentet, det hegeravas i Trun- 
cus og Halen. 
as to be seen from straight in front (fig. 7), its 
greatest breadth appears to be nearer the dorsal 
side, while the valves meet below in a rather sharp 
angle. The dorsal furrow, in this position of the 
shell, is like a deep hollow above. In the front 
part of each valve is seen, more or less distinctly, 
the round area of insertion of the adductor muscle 
of the shell, and immediately behind this, the shell- 
gland. The latter (see Pl. XIX, fig. 16) exhibits, in 
all essential particulars, the same structure as in 
Limnadia, but is comparatively shorter and broader. 
The surface of the shell is quite smooth, with- 
out a trace of lines of growth. It is true that in 
certain lights, what appears to be a faint, curved 
line is visible at some distance from the free edges 
of the valves (see Pl. XVIII, fig. 6); but I am con- 
vinced that this is no true line of growth, but is 
due to the circumstance that the marginal portion 
of the valves is of a somewhat different structure to 
the other part, a fact which has also been observed 
by Grube. In other respects the shell consists, 
as in Limnadia, of 2 lamelle, an external, fairly 
strong, but very elastic, chitinous lamella, and an 
internal extremely delicate membrane, which forms 
a continuation of the integument of the body. 
Under a very high magnifying power, the external 
lamella of the shell exhibits regularly rounded 
meshes or hollows. The edges of the valves are 
somewhat thickened, and perfectly smooth. 
The body itself (see Pl. XIX, fig. 1) is attached 
to the shell in exactly the same way as in Limnadia, 
namely, by a narrow dorsal ligament which runs 
inwards from the shell’s most prominent part, and 
at some distance below this, by the adductor muscle. 
The body is thus suspended in the cavity of the 
shell in the same way as in Limnadia, so that both 
the fore and the hind parts can be moved freely. 
In proportion to the shell, however, the body in the 
present form is much more voluminous than in Lim- 
nadia, and consequently, as a rule, a large portion 
of the head, both in living and in spirit specimens, 
projects from the shell in front (see also Pl. XVII, 
figs. 1 and 2). By a very great flexure of the head, 
however, whereby it is bent in against the ventral 
surface of the body, it can, like the rest of the 
body, be completely drawn into the shell, when the 
valves fit closely together all round. As in Lim- 
nadia, the body falls naturally into two principal 
sections, one anterior and one posterior, separated 
from one another by the dorsal ligament and the 
adductor muscle of the shell; but the proportion of 
these 2 sections is here quite different, the anterior 
one being fully as large as the posterior. Both sec- 
tions admit of sub-division into 2, the anterior into 
head and cervical segment, the posterior into trunk 
and tail. 
