87 



og kau aabnes luiget eller lukkes efter Dyrets 

 Behag. I sidstuævnte Tilfælile slutter deres frie 

 Kauter tæt mod iiinandpii, saa at Dyret er herme- 

 tisk indeshittet i >Skallens ('avitet. Skallen er t'æ- 

 stet til Legemet oventil ved et temmelig smalt 

 Ligament og noget nedenfor dette til hver Side ved 

 den stærke Ijukkemnskel, hvis Insertion til Inder- 

 siden af hver Valvel viser sig som et vel begrænd- 

 set rundagtig Felt i dennes forreste Del. Umiddel- 

 bart bag dette Felt sees Skalkjertelen strækkende 

 sig paaskraat bagover hver Valvel, Seet fra Siden 

 (Fig. 1) viser Skallen hos fnldt udvoxede Exempla- 

 rer en noget iiregelmæssig, bredt oval Form, med 

 den storste Hoide, der sædvanligvis o\-erstiger 'V* 

 af Længden, beliggende foran Midten. Dorsalkanten 

 er hos ældre Exemjilarer meget stærkt buet i sin 

 forreste Del og begrændset fortil ved en temmelig 

 utydelig stuni])\'inklet Afsats, bagtil ved et noget 

 stærkere fremtra'dende Hjorue. Nedenfor disse Hjør- 

 ner begynder A'alvlei'ues frie Kanter, og disse dan- 

 ner i hele sin Længde en uafbrudt og fuldkommen 

 ievn l>ue. Forenden af Skallen er meget kort og 

 afstum])et, medens Bagenden er noget nddraget og 

 smalt tilrundet. Paa Siderue af Skallen sees et 

 Antal meget tine concentriske Linier, de saakaldte 

 Væxtstriber. Deres Antal er varierende efter Al- 

 deren. Hos de storste af mig observerede Individer 

 har jeg talt 11 saadanne Striber paa hver A'alvel; 

 men Prof. Lilljeborg har paa ualmindelig store 

 Exem])larer fundet indtil lo Par Væxtstriljer. Alle 

 disse Stril)er convergerer mod det forreste Hjorne 

 af Skallen, og ligger folgelig her tæt sannnen, hvor- 

 imod de bagtil ender i forskjellig Hoide langs Dorsal- 

 kanten af Skallen. De yderste Linier, som ialmin- 

 delighed er tættere, lober nogenlunde parallelt med 

 Valvlernes frie Kanter; ved den inderste Linie lie- 

 grændses oventil et ovalt Felt, hvor Skallen har 

 sin storste Brede, og indenfor hvilket Insertifuis- 

 areaen for Skallens liukkemnskel og Skalkjeilelen 

 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 bet\'de- 

 lig mindre end ^/v af Ivængden, tV>ran Midten. 



Hvad Skallens finere Bygning angaar, saa viser 

 den en fuldkommen glat og glindsende Overtlade, 

 Ilden Spor af nogen ydre Skulptnr. r)en er imidler- 

 tid, ligesom Eygskjoldet hos Lepidurus, sammensat 

 af 2 væsentlig forskjellige Dele, en ydre, tilsyne- 

 ladende af tiere Lag bestaaende chitinos Skikt, og 

 en indre membranos Beklædning, der danner en 

 Fortsættelse af Legemets Integument. Begge disse 

 Skikter er forbnndne med hinandeu ved talrige 

 Tværbjælker, der delvis sees udvendigf som uregel- 

 mæssige opake Smaajiletter, (jg mellem hvilke der 



or sliut according to the jileasnre nf the animal. 

 In the latter case, their free edges tit closely toge- 

 ther, so that the animal is hermetically enclosed in 

 the shell's cavity. The shell is attached to the 

 body above l)v a rather narrow liganiMit, and a 

 little l)elo\v this, to each side, by the strong ad- 

 ductor niusrle. whose insertion <in tlie interior sur- 

 face of earh valve appears as a welldetined i-inailar 

 area in tlie foremost ])art of the valve. Imme- 

 diately Ijchiud this area, tlie shell-gland is seen 

 extending obliipiely backwards over each valve. 

 Seen from the side (tig. 1), the shell, in fully-grown 

 specimens, exhibits a rather irregular, broadly oval 

 form, with its greatest height, wliich generally ex- 

 ceeds ^li of its length, in front of the middle. The 

 dorsal margin in older animals is very much curved 

 in its foremost part, and boumled in front b\- a 

 somewhat indistinct, obtuse angled projection, lieliind, 

 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 ])erfectly even curve. Tlie anterior end 

 of the shell is very short and lilunt, -while the 

 other end is rather drawn out, and narrowh' rouiidfd. 

 On the sides of the shell, a number of vcrv tine 

 concentric lines are visible, the so-called lines of 

 growtlj. Their number varies according to age. In 

 the largest specimens examined by nie. I have 

 counted 11 such lines on each valve; but Prof. 

 Lilljeliorg luis found as many as If) jiairs <if lines 

 of growth on nnusuall\' 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 ditferent heights 

 along the doi'sal edge <if the shell. The outermost 

 lines, wliich 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 muscde, and the sliell-gland are situate<l. 

 Seen from above (fig. 2) or from below (fig. '■)), the 

 shell exhibits a very narrow, somewhat fusiform 

 .shape, with the greatest breadth, which is consider- 

 ably less than '/'- 'd' the length, in front of the 

 middle. 



As regards its more delicate structure, tlie shell 

 presents a perfectly smooth and shilling surface, 

 without a trace of any external sculpturing. It is, 

 however, like the carapace in Lcpiiliirns, composed 

 of two essentially difi'ercnt jiarts, 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, o[)a(|ue spots, and between which 



