126 G. LlNDTKuM, ON THE SILUKJAN GASTUOPODA AND l^TKKOroDA OF GOTLAND. 



It is very difficult to find luiy characteristics wherewith to distinguish those 

 forms called Murchisonia in u I'eady uuinner from Pleurotoniuria. According to the 

 authors of this genus the aperture should be »terniinee a sa base par un canal tres 

 court ou tronque». But in several Pleurotomariie the superior corner of the aperture 

 may also be seen to be protracted in an angle. Salter in Canad. Org. Remains, vol. I 

 p. 18 divides this genus in Murchisonia proper with acutely carinated whorls and 

 ilormotonia with beaded, rounded whorls and rounded aperture. But while then in 

 the former genus many Pleurotoniarije with broad spire liave been included, it is more 

 practical with Buonn to include all banded shells witli elongated and slender spire of 

 many whorls whether carinated and ornamented or plain, in Murchisonia and consider 

 it as a subgenus merely to Pleurotomaria. 



This genus occurs as early as in the Bala limestone of the Cambrian formation, 

 according to Salter, Catal. Gambr. Foss. p. 68, and continued through all Palteozoic 

 formations, while in the Mesozoic ones no such elongated Plcurotoinarite are known with 

 any degree of certainty. With us in Sweden they are scarce in the Lower Silurian, 

 a large species, related to the Esthonian M. insignis Eiciiw., having been found in a few 

 s|)ecimens at Grasgard and Segersta in Oland in the uppermost limestone beds. In the 

 Ui)per Silurian, again, of Gotland they are numerous and especially there are nuclei of 

 such elongately whorled forms, which also may be Loxonemata, filling the strata in 

 several places. Of some species the shell h;is always been destroyed and they are known 

 only by the impression of it in the rock. This is very strange as the shell of the nearly 

 related Pleurotomariai is often well preserved in the same strata. The slit band is built 

 upon the same plan as in Pleurotomaria. In Murchis. detlexa there is a peculiar devia- 

 tion, as described in detail further down, when the superior margin attains so large a 

 development that it, in its downward growth, hides the band. In M. attenuata the slit 

 band is changed, on the body whorl, into a ridge, on which the apex of the angular 

 transverse lines rests, quite as in the Euomphalidie. 



As there, at least in the first division of this genus, is a certain sin)ilar uni- 

 formity prevailing in the ornamentation of the shell, the position of the slit band, 

 the shape and the size of the whorls, these will be the chief characteristics for distin- 

 guishing the various species. The genus may be fitly divided in two groups. 



Divisio I. SIMPLICES. 



Ornamentation uniform of backwards directed stria.' meeting the generally large 

 slit band in an acute angle; |)Osition of the band and shape and size of the whorls 

 giving the characteristics. 



1. M. cingulata His. 



2. M. cava n. 



3. M. moniliformis n. 



4. M. obtusangula n. 



5. M. subplicata n. 



6. M. compressa n. 



