6o 



LAMELIJBRANCHIATA. 



South Iceland: 



Vestmannaeyjar beach i valve. 



— 10—15 fm. I spec. 



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

 [i\t the Faeroes the species has not been /oiiiid\. 



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



a. Modiolaria discors L. var. lavigafa Gray (PL III, figs. 4a — b): the shell somewhat compressed, 

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



b. Modiolaria discors L. var. snbstriata Gray (PL III, 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 Icrvigata is a high form, but even among the full-grown we meet with specimens 

 whicli are much more elongated than is usually the case. The typical Iirvigata does not have the 

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



In 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 siibsfriafa has the 

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

 lens ("Modiolaria Icrvis" Beck I.e.). 



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

 forms a given specimen has to be referred; in the case of snuil! specimens it is often quite impossible. 



Modiolaria Ice.-vigata-siibstriata are arctic and circumpolar forms, whose southern boundaries lie 

 at Massachusetts, Lofoten and northern Japan. Viewed in a wider sense, including also Alodiolaria- 

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

 Kattegat-Baltic (to Kiel and Neustadter Bugt) and the Mediterranean 2). 



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

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



And I am most inclined to believe with Jeffrews, that Modiolaria hcvigata Gray and Modio- 

 laria discors Linne are specificalh- 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 kevigata in the very young stages have the posterior area radially striated and in 

 the adult condition retain this striation on the umbonal region; the striation is obviously a characteristic of the 3'oung stages. 



=) Bucquoy, 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. 7narmornta Forb. has been confused with Mod. discors. 



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

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

 Iceland, Spitzbergeu, 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 

 (Tromsosundets moUuskfauna ; Tromso Museums Aarshefter VIII, 18S6, p. 65) the form snbstriata ("Mod.lctvis Beck") does not 

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



4) The maximum length of Mod. discors y^\. Denmark and Norway _) is 20"""., whereas Mod. substriata becomes over 50 "'m. long. 



