ON CAEBONIFEBOUS SPECIES OE MUECHISONIA. 



617 



42. Notes upon some Caebonifeeous Species of Muechisonia in our 

 Public Museums. By Miss Jane Donald, Carlisle. (Read 

 May 25th, 1887.) 



[Communicated by J. G. Goodchild, Esq., F.G.S., of H.M. Geological Survey.] 



[Plate XXIY.] 



It is exceedingly difficult to fix the limits of the genera of many 

 fossil Gasteropoda and to ascertain their affinities with those of 

 existing forms. As the soft parts of the animals are never preserved, 

 the only data upon which we can rely are such as may be derived 

 from the form and structure of the shell. The most characteristic 

 feature in the Murchisonice is the slit in the outer lip, the successive 

 filling-up of which gives rise to the formation of a band upon the 

 whorls of the shell. Now this slit in the outer lip exists in several 

 other genera, such as Pleurotomaria, Pleurotoma, some species of Tur- 

 ritella, Siliquaria, Scissurella, Emarginula, Bellerophon, and, to a 

 greater or less degree, also in Raliotis, Ianthina, Euomphalus, &c. Of 

 these genera, those which Murchisonia most resembles are Pleuroto- 

 maria and Turritella, and in some instances it is very difficult to 

 draw the line between the genus Murchisonia and the genus Pleuro- 

 tomaria ; in both genera the slit is represented on the whorls by a 

 well-defined band, and the only external difference between the 

 shells is the form of the spire, that of Murchisonia being elongated, 

 while in Pleurotomaria it is short. But there are shells of every 

 intermediate length, and these have been referred sometimes to the 

 one genus, and sometimes to the other. G. Lindstrom (' Silurian 

 Gasteropoda and Pteropoda of Gotland,' p. 92 *) thinks it well to 

 limit the genus Murchisonia to shells which have a long slender 

 spire of more than six whorls. 



The structure of the shell of Pleurotomaria seems to differ from 

 that of Murchisonia in the interior being nacreous, and G. Lindstrom 

 states that many of the Gotlandic Pleurotomaria} have this inner 

 pearly layer preserved, but he has not observed this structure in any 

 of the Murchisonice from the same beds. In his description of 

 M. compressa (p. 130) he says that " the nuclei have a glossy surface, 

 and look as if the interior walls of the shell had been porcellanous." 

 None of the British specimens of Murchisonia that I have examined 

 have shown any evidence of the internal character of the shell. 

 Indeed, from the manner in which most of these are preserved, it is 

 difficult even to ascertain the relative thickness of the shell ; in most 

 instances it is thin, but in others it appears decidedly thick, com- 

 pared with the size of the shell. This thickness may not have 

 existed in the original shell, for Mr. Etheridge informs me that a 

 thickening sometimes takes place when the test is replaced by 

 some mineral substance, as is frequently the case in the Palaeozoic 

 Gasteropoda. 



Struck by the external resemblance of the Murchisonice to the 

 * Kongl. Svenska Vet.-Akad. Handl. Bd. xix. (1881), no. vi. 

 Q. J.G.S. No. 172. 2u 



