87 



og kan aabnes iioget eller lukkes efter Dyrets 

 Behag. I sidstnsevnte Tilfælde slutter deres frie 

 Kanter tæt mod liinanden, saa at Dyret er herme- 

 tisk indeshittet i Skallens Cavitet. Skallen er fæ- 

 stet til Legemet oventil ved et temmelig smalt 

 Ligament og noget nedenfor dette til hver Side ved 

 den stærke Lnkkemnskel, hvis Insertion til Inder- 

 siden af hver Yalvel viser sig som et vel begrænd- 

 set rundagtig Felt i dennes forreste Del. Umiddel- 

 l)art bag dette Felt sees Skalkjertelen strækkende 

 sig paaskraat bagover hver Valvel Seet fra Siden 

 (Fig. 1) viser Skallen hos fuldt iidvoxede Exempla- 

 rer en noget uregelmæssig, bredt oval Form, med 

 den største Hoide, der sædvanligvis overstiger •''A 

 af Længden, beliggende foran Midten. Dorsalkanten 

 er hos ældre Exem}»larer meget stærkt buet i sin 

 forreste Del og begrændset fortil ved en temmelig 

 utydelig stumpvinklet Afsats, bagtil ved et noget 

 stærkere fremtrædende Hjørne. Nedenfor disse Hjer- 

 ner begynder Valvlernes frie Kanter, og disse dan- 

 ner i hele sin Længde en uafbrudt og fuldkommen 

 i evn Bue. Foreuden af Skallen er meget kort og 

 afstiimpet, medens Bagenden er noget uddraget og 

 smalt tilrundet. Paa Siderne af Skallen sees et 

 Antal meget fine concentriske Linier, de saakaldte 

 Væxtstriber. Deres Antal er varierende efter Al- 

 deren. Hos de største af mig øbserverede Lidivider 

 har jeg talt 11 saadanne Striber paa hver Valvel; 

 men Prof. Lilljeborg har paa ualmindelig store 

 Exemplarer fundet indtil 15 Par Væxtstriber. Alle 

 disse Striber convergerer mod det forreste Hjørne 

 af Skallen, og ligger følgelig her tæt sammen, hvor- 

 imod de bagtil ender i forskjellig Høide langs Dorsal- 

 kanten af Skallen. De yderste Linier, som ialmin- 

 delighed er tættere, laber nogenlunde parallelt med 

 Valvlernes frie Kanter; ved den inderste Linie be- 

 grændses oventil et ovalt Felt, hvor Skallen har 

 sin største 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 største Brede, der er betyde- 

 lig mindre end V» af Længden, foran Midten. 



Hvad Skallens finere Bygning angaar, saa viser 

 den en fuldkommen glat og glindsende Overflade, 

 uden Spor af nogen ydi'e Skulptur. Den er imidler- 

 tid, ligesom Rygskjoldet hos LepkJurus, sammensat 

 af 2 væsentlig forskjellige Dele, en ydre, til.syne- 

 ladende af flere Lag bestaaende chitinos Skikt, og 

 en indre membranos Beklædning, der danner en 

 Fortsættelse af Legemets Integument. Begge disse 

 Skikter er forbundne med hinanden ved talrige 

 Tværbjælker, der delvis sees udvendigt som uregel- 

 mæssige opake Smaapletter, og mellem hvilke 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 sui'- 

 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 'A 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, ol>tuse angled projection, liehind, 

 by a rather more sliarply 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 visilde, the so-called lines of 

 growth. Their number varies according to age. In 

 the largest specimens examined b}^ 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 iinusually 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 difterent 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 '/a of tlie 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 apparentl}^ 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 



