87 



og kan aaLiies luiget eller lul<]<es ei'ter Dyrets 

 Behag. I sidstnævnte Tilfældc sliittn- deres frie 

 Kanter tæt mod hiiianden. saa at Dyret er herme- 

 tisk indeshittet i Skallens Cavitet. Skal len er fæ- 

 stct I il Legemet oventil ved et teniinelig smalt 

 Jjigament og noget nedenfur dette til hver Side ved 

 den stakke l.nkkemnskel, hvis Insertion til Jnder- 

 siden at hver \'alvel viser sig som et \v\ l)egra'nd- 

 set rnndagtig Felt i dennes tori-esle Del. Uniiildel- 

 liart iiag dette Felt sees Skalkjei'telen stra'kkende 

 sig paaskraat hagover hver ^"alvid Seet fra Siden 

 (Fig. \) viser Skallen Ims 1'uldt udxnxedr Fxi^njjla- 

 rer en noget uregelmæssig, l)re(lt o\-al h'orni, med 

 den storste Hoide, der sæd vanligvis (j verstiger 'Vi 



at [ja'ngd 



iiraen, hen 



irgende foran i\Iidti 



I )orsalk'anten 



er hos ældre Exemjdarer meget sta'rkt l)net i sin 

 forreste Del og liegrfpndset fortil ved en temmelig 

 nt\delig st nnipvinklet Afsats, hagtil ved et noget 

 staM'kere fi'emtra'dende Hjorne. Nedenfor disse Hjor- 

 ner hegynder Valvlernes frie Kantei-, og disse dan- 

 ner i h(di> sin Længde (Mi natljrndf ng fiddlcnmmen 

 ievn line. Forenden af Skallen er meget kort og 

 afstnm]>et, medens Bagenden er noget nddraget og 

 smalt tilrundet. Taa Siderne ai' Skalh'n sees et 

 Antal meget line conceiitriske Liniei', de saakaldte 

 Væxtstrilier. Dei'es Antal er varierende efter Al- 

 dei'en. Flus de storste af mig ohserverede Individer 

 har jeg talt 11 saadanne Strilier |iaa hver Valvel; 

 men Trof. Lillj(d)oi-g liar ]>aa ualmindelig store 

 Exemplarer i'nndet indtil If) I'ar X'iextsti'iher. Alle 

 disse Striher convergerer mod det forreste Hjorne 

 af Skallen, og ligger folgelig her ta>t sammen, h\or- 

 imod de hagtil ender i foi'skjellig Hoide langs Doi'sal- 

 kanten af Skallen. De ydei'ste Linier, som ialinin- 

 delighed er tættere. lober nogenlnnde pai-alhdt metl 

 \'alvlernes ii-ie i\anter; ved den inderste Linie be- 

 gramdses oventil et ovalt Fidt. hvor Skallen har 

 sin storste Bi'ede, og indenfor Indlket insertions- 

 areaen for Skallens Lukkenuiskel og Skalkjert(den 

 har sin i'hids. Seet ovenfra (Fig. -) idler nedenfra 

 (Fig. y) viser Skallen en meget snml, noget ten- 

 dannet F'orm, med den storste Brede, der er hetvde- 



lig mindre end 



/■■' 



af La-ngden, loran JMidten. 



Hvad Sk-allens finere Bygning angaar, saa \-iser 

 den en fnldkommen glat og glindsende Overtlade, 

 nden S])or af nogen ydre Skulptur. l)eu er imidler- 

 tid, ligesom iiygskjoldet hos Lcpidio'iis. sammensat 

 af :? væsentlig forskjellige Dele, en ydre, tilsyne- 

 ladende af flere Lag bestaaende ehitinos Skikt, og 

 en indre membranos Beklædning, der danner en 

 Fortsa^ttelse af Legemets Integument. Begge di.sse 

 Skikter er foi-bundne med liinandeu ved talrige 

 Tværbjælker, der delvis see.s ndvendigt som uregel- 

 mæssigi! ojiake Smaa])letter, og mellem hvillce der 



or shut according to the ])leasure of the animal. 

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

 ther, so that the animal is hermetically enclosed in 

 the shells cavity. The shell is attached to the 

 body above hy a rather narrow ligament, and a 

 little below this, to each side, by the strong ad- 

 ductor niusele. whose in.sertion on the interior sur- 

 face of each valve appears as a well-detined circular 

 area in the furennist part of the valve. Imme- 

 diately behind this area, the shell-gland is seen 

 extending obli(|uely liacdiwards o\er each valve. 

 Seen from the side (tig. 1), the slndl. in full\--gr<iwii 

 specimens, exhibits a rather irregular, broadly oval 

 form, with its greatest height, wliicdi generally ex- 

 ceeds '/4 of its length, in ti-imt of the middle. The 

 dorsal nurrgin in older animals is verv much curved 

 in its foremost ])art, aiul bounded in front by a 

 somewhat indistinet, obtuse angled projection, behind, 

 by a rather more sharply projecting angle. Below 

 these angles Ijegin the free edges of the valve.s, 

 and these form throughout their length an uninter- 

 rupted and perfectly even curve. The anterior end 

 of the shell is very shoi-t and l)lunt, while the 

 other end i.s rather drawn out, and narrowly rounded. 

 On the sides of the sladl, a nundier of verv tine 

 concentric lines are \-isible, the so-called lines of 

 growth. Their nundier varies according to age. In 

 the largest specimens examined b\' me. I have 

 counted 11 such lines on each vahc; but I'rof. 

 Tjilljeborg has found as mau\' as 1") pairs of lines 

 of growth on unusually large specimens. All these 

 lines converge towards the anterior comer of the 

 slndl, and consei|ueiitl\' lie (dose together there; 

 while at the back they eml at different heights 

 along the dorsal edge (d' the shell The outermost 

 lines, which are generally (doser, run to a certain 

 extent parallel with tln^ free edges (d' the valves. 

 The intermost line bounds, aliove. an oval Held 

 where the greatest breailth of the shell occurs, and 

 within which the area of insertion of the shell's 

 adductor mns( le, and the shell-gland are situated. 

 Seen from above (Hg. '2^ or from below (tig. ."»), the 

 shell exhil)its a very nai'row, somewhat f\isiforni 

 sha]ie, with the greatest breadth, wliicli is consider- 

 al)ly less than '/:' of the length, in front of the 

 middle. 



As regards its nnire delicate structure, the shell 

 presents a j)erfectl\' snujotli and shining s\irface, 

 without a trace of any external sculiitiuiug. It is, 

 however, like the cai'a|)ace in LcpidnrKX, com))osed 

 of two essentiall\- different jiarts, an exterior chiti- 

 nous coating aiijiarently consisting (d' several layers, 

 and an internal nn-mbi-anous lining, forming a cou- 

 tinuation (d' the integument of the body. Both these 

 strata are connected with one another b\- numerous 

 crossbars, whi(h are ))artially visible externally as 

 small, irregular, o])a(|ue s])ots, and Ijetween which 



