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og kan aabnes noget eller lukkes efter Dyrets 
Behag. I sidstnevnte Tilfeelde slutter deres frie 
Kanter tet mod hinanden, saa at Dyret er herme- 
tisk indesluttet i Skallens Cavitet. Skallen er fe- 
stet til Legemet oventil ved et temmelig smalt 
Ligament og noget nedenfor dette til hver Side ved 
den steerke Lukkemuskel, hvis Insertion til Inder- 
siden af hver Valvel viser sig som et vel begreend- 
set rundagtig Felt i dennes forreste Del. Umiddel- 
bart bag dette Felt sees Skalkjertelen straekkende 
sig paaskraat bagover hver Valvel. Seet fra Siden 
(Fig. 1) viser Skallen hos fuldt udvoxede Exempla- 
rer en noget uregelmessig, bredt oval Form, med 
den storste Hoide, der seedvanligvis overstiger °/4 
af Leengden, beliggende foran Midten. Dorsalkanten 
er hos eldre Exemplarer meget sterkt buet i sin 
forreste Del og begreendset fortil ved en temmelig 
utydelig stumpvinklet Afsats, bagtil ved et noget 
sterkere fremtreedende Hjorne. Nedenfor disse Hjor- 
ner begynder Valvlernes frie Kanter, og disse dan- 
ner i hele sin Leengde en uafbrudt og fuldkommen 
jevn Bue. Forenden af Skallen er meget kort og 
afstumpet, medens Bagenden er noget uddraget og 
smalt tilrundet. Paa Siderne af Skallen sees et 
Antal meget fine concentriske Linier, de saakaldte 
Veextstriber. Deres Antal er varierende efter Al- 
deren. Hos de storste af mig observerede Individer 
har jeg talt 11 saadanne Striber paa hver Valvel; 
men Prof. Lilljeborg har paa ualmindelig store 
Exemplarer fundet indtil 15 Par Vextstriber. Alle 
disse Striber convergerer mod det forreste Hjorne 
af Skallen, og ligger folgelig her tet sammen, hvor- 
imod de bagtil ender i forskjellig Hoide langs Dorsal- 
kanten af Skallen. De yderste Linier, som ialmin- 
delighed er tettere, lober nogenlunde parallelt med 
Valvlernes frie Kanter; ved den inderste Linie be- 
grendses oventil et ovalt Felt, hvor Skallen har 
sin storste Brede, og indenfor hvilket Insertions- 
areaen for Skallens Lukkemuskel og Skalkjertelen 
har sin Plads. Seet ovenfra (Fig. 2) eller nedenfra 
(Fig. 3) viser Skallen en meget smal, noget ten- 
dannet Form, med den storste Brede, der er betyde- 
lig mindre end ‘/s af Lengden, foran Midten. 
* 
Hyvad Skallens finere Bygning angaar, saa viser 
den en fuldkommen glat og glindsende Overflade, 
uden Spor af nogen ydre Skulptur. Den er imidler- 
tid, ligesom Rygskjoldet hos Lepidurus, sammensat 
af 2 vesentlig forskjellige Dele, en ydre, tilsyne- 
ladende af flere Lag bestaaende chitinos Skikt, og 
en indre membranes Bekledning, der danner en 
Fortsettelse af Legemets Integument. Begge disse 
Skikter er forbundne med hinanden ved talrige 
Tverbjelker, der delvis sees udvendigt som uregel- 
messige opake Smaapletter, og mellem hyilke der 
or shut according to the pleasure of the animal. 
In the latter case, their free edges fit closely toge- 
ther, so that the animal is hermetically enclosed in 
the shell’s cavity. The shell is attached to the 
body above by a rather narrow ligament, and a 
little below this, to each side, by the strong ad- 
ductor muscle, whose insertion on the interior sur- 
face of each valve appears as a well-defined circular 
area in the foremost part of the valve. Imme- 
diately behind this area, the shell-gland is seen 
extending obliquely backwards over each valve. 
Seen from the side (fig. 1), the shell, in fully-grown 
specimens, exhibits a rather irregular, broadly oval 
form, with its greatest height, which generally ex- 
ceeds */s of its length, in front of the middle. The 
dorsal margin in older animals is very much curved 
in its foremost part, and bounded in front by a 
somewhat indistinct, obtuse angled projection, behind, 
by a rather more sharply projecting angle. Below 
these angles begin the free edges of the valves, 
and these form throughout their length an uninter- 
rupted and perfectly even curve. The anterior end 
of the shell is very short and blunt, while the 
other end is rather drawn out, and narrowly rounded. 
On the sides of the shell, a number of very fine 
concentric lines are visible, the so-called lines of 
growth. Their number varies according to age. In 
the largest specimens examined by me, I have 
counted 11 such lines on each valve; but Prof. 
. Lilljeborg has found as many as 15 pairs of lines 
of growth on unusually large specimens. All these 
lines converge towards the anterior corner of the 
shell, and consequently lie close together there; 
while at the back they end at different heights 
along the dorsal edge of the shell. The outermost 
lines, which are generally closer, run to a certain 
extent parallel with the free edges of the valves. 
The intermost line bounds, above, an oval field 
where the greatest breadth of the shell occurs, and 
within which the area of insertion of the shell’s 
adductor muscle, and the shell-gland are situated. 
Seen from above (fig. 2) or from below (fig. 3), the 
shell exhibits a very narrow, somewhat fusiform 
shape, with the greatest breadth, which is consider- 
ably less than */s of the length, in front of the 
middle. 
As regards its more delicate structure, the shell 
presents a perfectly smooth and shining surface, 
without a trace of any external sculpturing. It is, 
however, like the carapace in Lepidurus, composed 
of two essentially different parts, an exterior chiti- 
nous coating apparently consisting of several layers, 
and an internal membranous lining, forming a con- 
tinuation of the integument of the body. Both these 
strata are connected with one another by numerous 
crossbars, which are partially visible externally as 
small, irregular, opaque spots, and between which 
