LAMELLIBRANCHIATA. <,, 



of " Mod. substriata" , so small is the difference between them'). Mod. discors (PL III, figs. 6 a hi is on 

 the whole a more oblong form; this is the essential difference. But whilst maintaining, that Mod. Icevigata, 

 Mod. substriata and Mod. discors are identical specifically, I may yet emphasize that in their typical 

 shape they are representatives for just as many special forms and must not be summarily thrown together. 



Furthermore, the form discors, as I know it from Danish waters, is also not a little variable; 

 frequently the posterior area is distinctly striated, but sometimes it appears almost smooth (cf. var. 

 semilcevis Jeffreys, Brit. Conchol. II, p. 1271; I have even before me at this moment a specimen from 

 the Great Belt (Svendborg), which combines a striatiou such as we find in substriata with the form of 

 a typical Icevigata, or on the whole shows a great resemblance to a very young Mod. Icevigata. 



In conclusion I may quote some little known observations regarding the biology of this sp< 



In his paper "Om Ueeus-Muslingen" (i.e. J/, discors var. Icevigata and substriata) ( >. Fabricii 

 writes as follows: "This Bivalve I have found to be of common occurrence in Greenland, where it is 



called Bibibiarsuk; I do not know the certain origin of this word one might be inclined to think, 



that the Greenlauders have found this name suitable for the whistling or hissing noise, this Bivalve... 

 produces when it has stood for a long time closed at ebb-tide for want of water and then with the 

 coming of the flood begins to open again. It is attached by its fine silk (byssus) to large stones or 

 rocks in the sea, the largest end sticking upwards, the open side turned towards the stone or foreign 

 body, to which it is attached. It lives preferably so far from the land, that it does not become dry 

 except at spring-tides when the greatest ebb occurs. On the blind rocks out to sea, therefore, it is 

 most numerous .... When its silky hairs have been broken by the waves or other cause, 1 have seen 

 it stretch out its foot, attaching it to the solid rock, elongating and contracting it, and thus push 

 itself forwards .... the fattest and largest are found on clay bottom " 



In a manuscript left by H. P. C. Moller we find: "It occurs everywhere in quantities on the 

 coast of Danish West Greenland and especially where there is shallow water, partly on Laminaria, 

 more rarely on Fucus, partly among the string-like algal forms and among stones, down to a depth 

 of 20 fm. or still deeper, e.g. at Nenuortalik, Godhavn; I have taken it in 30 fm. opposite Nepisene 

 at a distance of 8 Danish miles from the coast." 



And in a notice: "Ueber Mytilus discors" lap. Steenstr up makes the following remarks on 

 Modiolaria discors var. Icevigata at Iceland: ". . . . Noch mehr zeichnet sich diese Muschel dnrch ihren 

 sehr ausstreckbaren Fuss aus, welcher ihr erlaubt, die Byssusfaden nicht nur urn die Seiteu der Schalen, 

 soudern auch iiber den Rucken derselben herumzuf uhren ; dadurch kann sie sich ganz niit eiuei 

 Byssushiille umgeben oder sich wie in eiuem Byssussack verstecken. Die Enden der ausseren Bys- 

 susfaden werden auf kleinen Steinchen, Muschelschaleutrummern und dergleichen festgeheftet, so dass 

 der Byssussack ganz einem Steinhaufchen gleicht; die innere Wand dei Byssushiille ist dagegen sehr 

 glatt, aus dichtliegenden, sich kreuzenden Byssusfaden gebildet, und schliesst sich den Muschelschalen 

 ziemlich dicht an. Nur das hintere Ende der Hulle hat cine < )effnung, sonst ist sie ganz geschlossen. 

 In seichten Meeresbusen, in welcheu der Boden niit grobein Sande und Grande bedeckt war, hat S. 



') Lechc has obviously been of the- same opinion, as he writes (I.e. p. (51), that the .1/ Kiel Bay, 



sent him by MSbius, cannot lie specifically distinguished from Mod. larv - Beck; Leche refers to this 111 connection with the 

 fael, that G.O.Sars had erroneously identified Mobius' Mod. discors with I.' i Stimpson. 



z ) Nye Samling af det Kgl. Danske Videnskabernes Selskabs Skrifter, ,;. Deel, [78S, p. 153. 



