KONGL. SV. VET. AKADEMIENS HANDL. BAND. 19. N:0 6. 31 



as yet has becii ascertained. A few tubes from the shale beds of Wisby and Öster- 

 garn might possibly once have appertained to that group, but their iinperfect condi- 

 tion precludes Identification. 



As genera especially characteristic of the Sihirian beds of Gotland, raay be men- 

 tioned: Craspedostoma, Onychochilus and probably also Pycnomphalus, though it is 

 not certain whether the last does not occur in England and Oesel, but referred to the 

 genus Platyschisma in a coneeption Avhich is not that of the first author. Autodetus, 

 which is spread över the whole island, occurs also on the neighbouring Oesel, though it 

 is uncertain whether so coinmon there as on Gotland and not recorded from any other 

 Silurian region except Scania. As characteristic of the Upper Silurian formation in 

 general inay be annotated Chelodes, which has also been found in Bohemia, Tremano- 

 tus, Cyrtolites and Subulites also known from N. America and Canada, Subulites also 

 from England, and Tryblidium found in Estland and Canada. 



Concerning the many palaäozoic genera, of which I have appended an Index in 

 the end of this memoir, it depends much on individual opinions, as to what genus these 

 old fossil shells are to be referred, but it is evident that it is highly futile to establish 

 the genera only on a single and variable character. When comparing large numbers of spe- 

 cimens belonging to the same species and collected in the same locality the great variabi- 

 lity in the form of the aperture, the columella, the height of the spire, the umbilicus, will 

 be evident, and it is only necessary to remind of such forms as Platyceras cornutum 

 to discover the wide range within Avhich some species vary, chiefly those Avhich have 

 a large horizontal and vertical distribution. In almost the same degree Cyclonema 

 delicatulum varies AAdth high or Ioav spire, slender or ventricose Avhorls, Avith open or 

 closed umbilicus. 



Among structural peculiarities ought to be observed that in the genus Euom- 

 phalus, as it has been long known, the shell, in the rule, near the apex or in the 

 oldest whorls, is divided into one or several compartments or chambers by concave 

 diaphragms, Avithout, however, having any communication or sipho betAveen thera. The 

 charabered apex has very often through the fossilisation become deciduous and the 

 blunt end indicates the place, Avhere the partition once traversed the shell. In Murchi- 

 sonia, Loxonema and a presumed Trochus, Tr. gotlandicus, the apex is tilled Avith a 

 compact mäss of calcareous matter quite as in the recent Magilus. 



The peculiar and cellulär structure of the external Avalls in Autodetus is a feature 

 Avell deserving attention. In a few genera, as Platycex'as, Pleurotomaria, Euomphalus, 

 there is a tendency in several species to form scalarid varieties and in some species 

 all specimens found have assumed that peculiarity in groAvth. In the large genus 

 Pleurotomaria it is easy to folloAv the morphological changes AA^iich the characteristic 

 slit band is subject to, from a concave groove to the large, thin lamina in the group 

 Alata3. In respect to the sculpture of the surface I have called the spiral lines, keels 

 and ridges, Avhich run from the apex to the aperture for longitudinal, as they follow 

 the shell in its Avhole length, and, consequently, those intersecting them are to be 

 denominated transverse. Some authors call all the låter for lines of growth. But 

 besides the real lines of growth or the old apertural lips, Avhich are conspicuous by 



