126 G. LINUTRÖM, ON TIIE SILUKIAN GASTliOPODA ANIJ PTKKOFODA OF GOTLAND. 



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

 forins called Murchisonia in a ready manner from Pleurotoinaria. According to tlie 

 authors of this genus the apertiire sliould be »terininée k sa base par un canal tres 

 court ou tronqué». But in several Pleurotoiuarias the superior corner of the aperture 

 inay also be seen to be pi'otracted in an angle. Saltek in Canad. Org. Rernains, vol. I 

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

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

 the former genus many Pleurotomaria^ with broad spire have been included, it is more 

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

 inany whorls whether carinated and ornaraented or plain, in Murchisonia and consider 

 it as a subgenus merely to Pleurotoraaria. 



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 Pala3ozoic 

 formations, while in the Mesozoic ones no such elongated Pleurotomariaj 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 EiCHw., having been found in a few 

 specimens at Gräsgård and Segersta in Öland in the uppermost limestone beds. In the 

 Upper 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 has always been destroyed and they are known 

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

 related Pleurotomaria3 is often well presei-ved in the same strata. The slit band is built 

 upon the same plan as in Pleurotomaria. In Murchis. deflexa 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 Euomphalidaa. 



As there, at least in the first division of this genus, is a certain similar 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. 



ö 



Divisio I. SIMPLIOES. 



Ornamentation uniform of backwards directed striaj meeting the generally large 

 slit band in an acute angle; position 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. 



