280 



M. D'Archiac on the Genus Murchisonia. 



In fact, the Murchisonue cannot be confounded with the 

 Schizostoma, as the latter are depressed, planorbular, with a 

 very wide umbilicus, and without a columella ; they are, in fact, 

 true Euomphali with a notched aperture. The charac- 

 ters of the aperture of Melania equally oppose the introduc- 

 tion of Murchisonia into that genus ; and it differs from Pleu- 

 rotomaria by its turrited and more elongated form, as well as 

 in the aperture, which authors do not appear to have suffi- 

 ciently noticed hitherto. The absence of a true canal, and the 

 slight elongation of the columella, which is curved, equally 

 oppose its introduction amongst the Pleurotomce. The fades 

 of Murchisonia distinguishes it at the first glance from Turri- 

 tella. The disproportion of the successive whorls in some 

 varieties, and the irregularities observable in the growth of 

 others, are doubtless empirical characters, but exceedingly 

 common in Murchisonia and Cerithium, whilst they are rare 

 in Turritella, which is perfectly regular throughout the whole 

 length of the spire. The external ornaments of Murchisonia 

 are also more like those in Cerithium than Turritella. Lastly, 

 the examination of the mouth, the most important character 

 of all, removes Murchisonia still further from the latter genus. 

 Instead of being rounded, as in Turritella, it is oblong and 

 twice as long as wide, terminating at the lower angle in a 

 very small canal, and sometimes presenting a groove at the 

 upper angle. The columella, which is slightly curved, is re- 

 flected in the form of an S, and the right lip has a deep nar- 

 row notch with parallel edges. 



During the life of the animal the continuous closing of this 

 notch has produced an elevated keel or carina, which is sim- 

 ple or double, or rather a flattened fillet bordered by two 

 more or less elevated regular bands, approximating so nearly 

 in some species that they appear to unite, were it not for a 

 fine stria which enables one to distinguish them*. The 



* The notch in Murchisonia and Pleurotomaria is not closed in the same 

 manner as the sinus in Pleurotoma and some of the Cerithince. In the 

 former, the process of closing is in some measure independent of the growth 

 of the rest of the aperture. The striae are interrupted at the fillet or keel, 

 and those we observe on this part do not correspond with the inflected striae 

 above and beneath them. On the contrary, in Pleurotoma the sinus and 

 the aperture are closed by a continuous calcareous plate or lamina, and the 

 striae, though more or less inflected, are always uninterrupted ; in Pleuro- 

 tomaria and Murchisonia, the trace of the notch corresponding to that part 

 of the mantle of the animal, which, instead of a simple rounded hollow, 

 would present a deep narrow notch with parallel edges, is always distinctly 

 defined by two bands or striae, nothing similar to which appears in Pleuro- 

 toma or Cerithium. 



In variety e. of Murchisonia bilineata, nob., in Pleurotomaria limbata, 

 Phill., and P, Dcfrancii, nob., as well as in Schizostoma radiata, nob., it 



