6q T.AMEIJ.IBRANCHIATA. 



South Iceland: 



Vestmannaeyjar beach i valve. 

 10—15 fm. 1 spec. 



These specimens belong to the variety substriata and reach a length of 18 """. 

 I At the Faeroes the species has not been found]. 



Remarks. At Iceland and Greenland the species occurs, as mentioned above, under two main forms : 



a. Modiolaria discors L. var. Icevigata Gray (PI. Ill, figs. 4a — b): the shell somewhat compressed, 

 posteriorly high, rounded-truncate; the posterior area without radiating striae. 



b. Modiolaria discors L. var. substriata Gray (PI. Ill, figs. 5a — b): the shell ventricose, posteriorly 

 low, rounded-pointed; the posterior area with radiating striae. 



But each of these forms again is subject to variation. 



The typical Icevigata is a high form, but even among the full-grown we meet with specimens 

 which are much more elongated than is usually the case. The typical Icevigata does not have the 

 radiating strise on the posterior area, but sometimes a faint striation may be seen here 1 ). 



I11 the typical substriata the postero-dorsal margin slopes rapidly downwards, but we also find 

 specimens in which the slope of the posterior margin is less abrupt. The typical substriata has the 

 posterior area distinctly striated radially, but sometimes the striation is not to be seen, even under a 

 lens ("Modiolaria Icevis" I>eck I.e.). 



The two forms frequently occur together and one is often at a loss to determine to which of the 

 iui ins a given specimen has to be referred; in the case of small specimens it is often quite impossible. 



Modiolaria Icevigata-substriata are arctic and circumpolar forms, whose southern boundaries lie 

 at Massachusetts, kofoten and northern Japan. Viewed in a wider sense, including also Modiolaria 

 discors, the species is distributed further along the rest of West Europe to Madeira, including the 

 Kattegat-Baltic (to Kiel and Neustadter Bugtl and the Mediterranean-'). 



In opposition to G. O. Sars I must maintain, that Modiolaria substriata and Mod. Icevigata are 

 not distinct, but forms of one and the same species. 



And I am most inclined to believe with Jeffreys, that Modiolaria laevigata Gray and Modio- 

 laria discors Linne are specifically identical. Against this G. O. Sars has objected very definitely 

 and I can in so far agree with this author, that there is not an excessively great resemblance between 

 Mod. discors and an adult, typical Mod. Icevigata; on the other hand, I certainly consider it as more than 

 probable, that Mod. discors is nothing else but a southerns), pygmy t) and slightly altered form 



') I exclude here the fact, that all Icevigata in the very young stages have the posterior area radially striated and in 



the adult condition retain this striation on the unibonal region; the striation is obviously a characteristic of the young stages. 



.'■ icquoy, Dautzenberg and Dollfus however write in their oft-cited work, on the marine Mollusca of 



Roussillon, that the occurrence of Mod.discors in the Mediterranean requires to be confirmed; they mention various cases, in 



which Mod. marmorata Forb. has been confused with Mod. discors. 



I discors is certainly very commonly regarded as an arctic form, but the basis for this is undoubtedly a con- 

 fusion with the form substriata; among many hundreds of specimens, which I have examined from arctic regions (Greenland, 

 Iceland, Spitzbergen, Kara Sea), I have not found a single one of the true discors; this hardly goes further than northernmost 

 Norway or Murman Coast, further east and north it is replaced by the substriata form. As mentioned by Sparre Schneider 

 nsosundets ruollusk fauna; Tromso Museums Aarshefter VIII, 1SS6, p. 65) the form substriata ("Mod. lariiis Heck") does not 

 occur at Norway for the simple reason, in my opinion, that Mod. discors is its modification in boreal regions. 



) The maximum len thol \fod discors (at Denmark and Norway) is 20""".. whereas Mod. substriata becomes over 50 ""». long. 



