KONGL. SV. VET. AKADEMIENS HANDLINGAR. BAND. 19. N:0 6. 145 



Fam. VIII. TROCHIDiE D'Orb. 

 Gen. TR0CHU8 L. 



1758 Troclms L. Syst. Nat. ed. X, 756. 



1879 Palceotroclms J. Hall Pal. N. York vol. V pt. II, 133. 



1881 Flemmgia De Koninck Paune II, iii, 94. 



1882 Eotroclms Whitfield Bullet. N:o 3 of American Museum of Natural History p. 77. 



It is with great diffidence the following species have been described as belonging 

 to the old genus Trochus. The only reason for placing them there is the general 

 exteriör shape of the shell, it having not been possible to find any evidences from the 

 microscopic structure of the shell nor from any traces of a nacreous stratum or an 

 operculum. On the other hand, there are so many genera of shells which have per- 

 sisted from the Silurian age through all the following and still continue, and it may 

 therefore not be thought an impossibility that also the Trochi existed already in the 

 Silurian times. Moreover, it is generally adopted that species of Trochus have been 

 found in the Devonian rocks, where they, however, as well as in the Carboniferous 

 liinestones, not are numerous. Most of these palaeozoic Trochi are nearest allied to 

 that division, where the inner lip is thin or only slightly thickened, and the umbilical 

 side flat. A few, as Tr. profundus and Tr. cavus ') evidently belong to a section of 

 Palasozoic Trochi, out of which Whitfield (1. c.) has established his genus Eotrochus. 

 They have exactly the same cup shaped conformation of the umbilical side, which has 

 led some authors ^) to place similar ones from the Jurassic formations with the Phoridaa 

 in the genus Onustus. I think, however, that the Palaäozoic may, at least for the 

 present, be retained in Trochus. On comparing the beautiful figures of Tr. lamellosus 

 for instance in D'Orbigny's Pal. Fran^aise, Terr. Jurass. pl. 311 fig. 11 — 13, the re- 

 markable similarity with the Silurian Tr. cavus and profundus is evident. There is in 

 all the same cup shaped umbilical surface, the thin, lameilar börder around it and the 

 open umbilicus. There are no traces of any nacreous stratum. The columella, probably 

 solid in some, is in many replaced by a wide, open, funnel shaped axis. The apex of 

 the whorls is filled up with a calcareous deposit. 



Sixteen species have been discovered in the limestone beds of Gotland, none 

 having hitherto been found in the subjacent shales. These species may be divided 

 into the following groups. 



I. TRANSVERSI, transversally ornamented by oblique lines. 



1. Tr. Gotlandicus n. 



2. Tr. fulminatus n. 



3. Tr. mollis n. 



') Probably also Tr. Stuxbergi and imd ulans. 



^) HuDLESTON Contributions to the Pal. of the Yorkshire Oolites. Geol. Mag. 1884, p. 293. 



K. Sv. Vet. Akad. HaocU, Bd. 19. N:o 6. ' 19 



