KONGL. SV. VET. AKADEMIENS HANDL. BAMU. 19. N:0 6. 93 



(nat. size and luagiiiiied) sectioned transversally and the lamellu) of growth as in tlic 

 former species, riblike with sniooth interstices, directed obliquely outward, towards the 

 aperture of the shell. In hg. oU the slit band is sectioned along its whole breadth in 

 a Ii ne from the whorl outwards, intersecting some ten lamella^ of growth obliquely. 

 Section lig. ol is taken parallel to tlie exteriör börder, showing the apices of eleven of 

 the elongated crescentshaped lines of growth. The same large development also obtains 

 in some other species which form the division »Alata;» and of which several instances 

 are given on plate XI. This extraordinary increase of the slit band might at the first 

 look be regarded as a derived feature that attained such dimensions in the Upper 

 Silurian times. But on finding shells as low down in the series as in the Lower 

 Gray Orthoceratite limestone with a quite identical slit band one might as well ask 

 whether there not has been a regress from the development of this band to what we 

 find prevailing in a reduced shape during låter geological periods- As we are not in 

 possession of materials ample enough from still older strata, we are not able fairly to 

 build any conclusion on this point. The aliform slit band is easily distinguished from 

 other lamellar expansions, such as are found in some Trochi and others, by a section, 

 through which the interiör compartments become visible, whereas in the latter shells 

 the interiör is compact. 



The character of the slit band varies in the selfsame specirnen of some species, 

 as for instance in Pl. elliptica and it changes its nature and shape during continued 

 growth quite as is the case with the Bellerophons. 



Another peculiarity amongst some of the Pleurotomaritc is that the apex has been 

 either filled up with a solid mäss of calcareous matter or has been subdivided in se- 

 veral small compartments through imperforated diaphragms or tabuUx;. This peculiarity 

 they share with Murchisonia, Loxoncma and Euomphalus, amongst which it is common. 

 At the same time this tendency is often accompanied by the formation of a scalarid 

 shell. 



Species of Pleurotomaria are already found in the Lower Red Orthoceratite lime- 

 stone of Sweden, beginning with Pleurotomaria qualteriata. Pleurotomaria elliptica 

 HisiNGEii also is found in the Upper Gray Orthoceratite Limestone of Sweden. But it 

 was in the Upper Silurian seas, in the strata, which now make up the chief mäss of 

 Gotland that this genus attained a considerable development in no less than 39 species 

 and varieties of changing aspect and ornamentation. These species may most con- 

 veniently be subdivided in groups, according to their agreement as to the chief cha- 

 racteristic, the slit band, along with which feature several peculiarities follow in the 

 general form of the shell, its ornamentation, the position of the band on the whorls 

 etc. I propose then to range the Gotlandic species in the following groups taking the 

 form and ornamentation of the slit band as the principal ground of division. 



I. MULTICARINAT.^. Slit band accompanied on each side by one or two lines, 

 crescents crossed by several longitudinal lines. 



1. Pl. scutulata n. 



2. Pl. gradata n. 



