119 



at mail faar se den lige fovtVa (Fig. 7), viser den 

 sig at have sin storste Brede nærmere Dorsalsiden, 

 medens A'alvleriie nedad stoder sammen under en 

 temmelig spids Vinkel. Den dorsale Fure viser sig 

 i denne Stilling af Skallen som en dyb Indbugtning 

 oventil. I den forreste Del af liver Valvel sees 

 mere eller mindi'e tydeligt den rundagtige Inser- 

 tionsarea for Skallens Lukkemuskel og umiddelbart 

 bag denne Skalkjærtelen. Denne sidste (se Tab. 

 XIX, Fig. 16) viser i alt væsentlig samme Bygning 

 som hos Linmadia, men er forholdsvis kortere og 

 bredere. 



Skallens Overflade er ganske glat, uden Spor 

 af Yæxtstriber. Eigtignok sees under visse Belys- 

 ninger, i nogen Afstand fra Valvlernes frie Kanter, 

 ligesom en svag buet Liiiie (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 Structur end den ovrige Del, noget 

 der ogsaa er bemærket af Grube. Skallen liestaar 

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

 temmelig stærk, men meget elastisk chitinos La- 

 melle, og en særdeles delikat indre Membran, der 

 danner en Fortsættelse af Legemets Integument. 

 Ved meget .stærk Forstorrelse viser Skallens ydre 

 Lamelle regelmæs.sige rundagtige Masker eller For- 

 dybninger. Kanterne af Valvlerne er noget fortj'k- 

 kede og fuldkommen glatte. 



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

 til Skallen paa en fuldkommen lignende Maade som 

 hos Limnadia, nemlig ved et smalt dorsalt Ligament, 

 der udgaar indad fra Skallens mest fremspringende 

 Del, og i nogen Afstand under dette ved Adductor- 

 muskelen. Legemet er paa denne Maade ojihængt 

 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 nær\'ærende Form meget 

 mere voluminøst end hos Limnadia, hvorfor ogsaa i 

 Regelen, saavel hos levende som Spiritus-Kxemplai-er, 

 en storre Del af Hovedet rager frem af samme for- 

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

 meget stærk Boining af Hovedet, hvorved dette 

 lægges ind mod Bugtladen af Ki'oppen, kan dog 

 dette, ligesom det ovrige Legeme, helt inddrages 

 indenfor Valvlerne, som da slutter tæt sammen 

 overalt. Som hos Limnadia, falder Legemet natur- 

 ligt i 2 Ho\'edafsnit, et forreste og et bagerste, 

 begrændsede fra hinanden ved det dorsale Ligament 

 og Skallens Lukkemuskel: men Forholdet mellem 

 disse 2 Afsnit er her et helt andet, idet det forre- 

 ste er fixldkommen ligesaa stort som det bagerste. 

 Begge Afsnit lader sig igjen dele i 2, det forreste 

 i Hovedet og Nakkesegmentet, det bagerste i Trnn- 

 cus og Halen. 



as to be seen from straight in front (lig. 7), its 

 greatest breadth ajipears 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 deen hollow above. In the front 

 part of each valve is seen, more or less distinetl}', 

 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 

 Limnadia, but is comparatively shorter and Ijroader. 



The surface of the shell is rpiite smooth, with- 

 out a trace of lines of growth. It is true that in 

 certain lights, what appears to be a faint, curved 

 line is visible at some distance from the free edges 

 of the valves (see PI. XVIII, fig. 6).- but I am con- 

 vinced that this is no true line of growth, but is 

 due to the cirrumstance 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 G-rirbe. In other respects the .shell consists, 

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

 .strong, but very elastic, chitinous lamella, and an 

 Internal extremely delicate membrane, which forms 

 a continuation of the integument of the body. 

 LTnder a \gvj high magnifying power, the external 

 lamella of the shell exhibits regularly rounded 

 meshes or hollows. The edges of the valves are 

 somewhat thickened, and perfectly smooth. 



The body itself (see PI. XIX, fig. 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 part, and 

 at some di.stance below this, by the adductor muscle. 

 The body is thus suspended in the ca\'ity of the 

 shell in the same way as in LJmnadia, so that both 

 the fore and the hind parts can be moved treely. 

 In proportion to the shell, however, the body in the 

 present form is much more voluminous than in LJm- 

 nadia, and consequently, 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). By a very great flexure of the head, 

 however, whereby it is bent in against the ventral 

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

 body, be completely 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 here (juite difi'erent, the anterior 

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

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

 head and cervical segment, the posterior into ti'unk 

 and tail. 



