Hi) 



at man faar se den lige fovfra (Fig. 7), viser den 

 sig at have sin storste Bi'ede nærmere Dorsalsiden, 

 medens A^alvlerne nedad stoder sammen under on 

 temmelig spids Vinkel. Den dorsale Fure viser sig 

 i denne Stilling af Skallen som en dyh Indbugtning 

 oventil. I den forreste Del af hver Valvel sees 

 mere eller mindre tydeligt den rundagtige Inser- 

 tionsarea for Skallens Lukkemuskel og umiddelliart 

 bag denne Skalkjærtelen. Denne sidste (se Tab. 

 XIX. Fig. Ki) viser i alt væsentlig samme Bygning 

 som lios Limnat/ia, men er forholdsvis kortei'e og 

 bredere. 



Skallens OverHade er ganske glat, uden Spor 

 af Væxtstriber. 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 overbevist mig om, at dette ikke er 

 nogen virkelig Væxtstribe, men .skriver sig fra den 

 Omstændighed, at Valvlernes Randparti viser en 

 noget anden Struetur end den ovrige Del, noget 

 der ogsaa er bemærket af Grube. Skallen be.staar 

 forovrigt, som hos Limnadia, af 2 Lameller, en ydre 

 temmelig stærk, men meget elastisk ehitinos La- 

 melle, og en særdeles delikat indre Membran, der 

 danner en Fortsættelse af Legemets Integument. 

 Ved meget stæi"k Forstørrelse viser Skallens ydre 

 Lamelle regelmæssige rundagtige Masker eller For- 

 dybninger. Kanterne af Valvlerne er noget fortyk- 

 kede og fuldkommen glatte. 



Selve Legemet er (se Tab. XIX, Fig. 1) fæstet 

 til Skallen paa en fuldkommen lignende ]\Iaade som 

 hos Limnadia, nemlig ved et smalt dorsalt Ligament, 

 der udgaar indad fra Skallens mest fremspringende 

 Del, og i nogen Afstaiul under dette ved Adductor- 

 rau.skelen. Legemet er paa denne Maade ophængt 

 paa en lignende Maade .som hos hin Slægt i Skal- 

 lens Hule, saaledes at baade den forreste og bager- 

 ste Del er frit bevægelige. I Forhold til Skallen 

 er imidlertid Legemet hos nærværende Form meget 

 mere voluminost end hos Limnadia, hvorfor ogsaa i 

 liegelen, saavel hos levende som Spiritus-Exemplai-er, 

 en større Del af Hovedet rager frem af samme fer- 

 til (se ogsaa Tab. XVIII, Fig. 1 og 2) Ved en 

 meget stærk Boining af Hovedet, hvorved dette 

 lægges ind mod Bugtladen af Krojipen, kan dog 

 dette, ligesom det oNTige Legeme, lielt inddrages 

 indenfor Valvlerne, som da slutter tæt sannnen 

 overalt. Som hos Lininadia, falder Legemet natur- 

 ligt i 2 Hovedafsnit, et forreste og et bagerste, 

 begrændscde fra hinanden ved det dorsale Ligament 

 og Skallens Lukkemuskel; men Forlidldet melleni 

 disse 2 Afsnit er lier 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 Nakkesegraentet, det bager.ste i Trun- 

 ens <ig 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 PI. XIX, fig. 16) exhibits, in 

 all essential particulars, the same structure as in 

 Lhnnadia^ 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 Ije a faint, curved 

 line is visible at some distance from the free edges 

 of the valves (see PI. XVIII, fig. tj); 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 fxriibe. In other respects the shell consists, 

 as in Limnadia, of 2 lamellæ, an external, fairly 

 strong, but ver\- elastic, chitinons 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 tliickened, and perfecth^ smooth. 



The body itself (see PL XIX, tig. 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 ])art, and 

 at some distance Ijelow tliis, 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 ])arts can be moved freely. 

 In proportion to the shell, however, the bod}- in the 

 present form is mucli more voluminous than in Lim- 

 nadia, and conse([uently, as a rule, a large portion 

 of the head, both in living and in spirit specimens, 

 projects from the shell in front (see also PI. XVIII, 

 figs. 1 and 2). )i\ a very great flexure of the head, 

 however, whereby it is Ijent in against the ventral 

 surface of the body, it can, like the rest of the 

 body, be com])letely 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 liere ipiite different, the anterior 

 one being fully as large as the jiosterior. Both sec- 

 tions admit of sub-division into 2, the anterior into 

 head aiul cervical segment, the posterior into trunk 

 and tail. 



