M. D'Archiac on the Genus Murchisonia. 



279 



appear to be yet determined, also presents as an important 

 character the notch on the right lip. 



We have thus many shells which, though similarly pro- 

 vided with a sinus, differ much in other respects ; for instance, 

 between the genera Schizostoma and Pleurotoma there is a 

 distance similar to that which separates Fusus from Euom- 

 phalus or Solarium, and there is as much reason for uniting 

 the former as the latter. We are aware, that to arrive at cor- 

 rect classification a combination of all the characters is neces- 

 sary, and that the attention must not be confined to one alone ; 

 it does not however appear that Count Munster has observed 

 this rule, for in his last work he gives the generic name Schi- 

 zostoma to many species not only widely differing from those 

 on which the genus was founded, but also varying considera- 

 bly from each other*. 



The shells which M. Verneuil and myself propose to unite 

 under the name of Murchisonia are widely distributed in the 

 formations beneath the coal series ; but we are not as yet 

 acquainted w T ith any above these formations f. Goldfuss at 

 first regarded many of them as Melanice, and afterwards as 

 Turritella ; Phillips and Hisinger have adopted the latter 

 name for other species ; Count Munster placed some of them 

 in the genus Schizostoma ; Murchison included two in Pleu- 

 rotoma and one in Pleurotomaria ; Von Buch, when descri- 

 bing Turritella cingulata (Hisinger), classes it as a Pleuro- 

 tomaria ; lastly, Beck is inclined to refer the Turritellce of 

 Goldfuss to the Cerithince. We shall now endeavour to show 

 whether this difference of opinion does not sufficiently prove 

 that these shells, which we place between Cerithium and 

 Turritella, do not constitute a distinct group with sufficiently 

 marked characters to form a genus, or at least a subgenus, 

 in a systematic classification. 



* The genus Pleurotomaria appears, on the whole, to be ill defined, for 

 we find placed in it indifferently turbinated shells provided with a columella 

 and a small umbilicus with a quadrangular aperture, like Trochus, or 

 rounded, as in Turbo, and others which are discoidal, without a columella, 

 having the umbilicus sufficiently open to allow the whorls of the spire to be 

 seen, and an opening, which, joined to their other characters, gives them 

 the appearance of Solarium. We think with Bronn, that all species pre- 

 senting the latter characters ought to be placed in the genus Schizostoma, 

 as they differ as much from the others as Solarium does from Trochus or 

 Turbo. 



f If we had not personally procured well-preserved specimens of Turri- 

 tella concava, Sow. tab. 565, in the quarry at Chilmark (Wiltshire), the 

 figure given by this author would lead us to believe that this shell might 

 belong to the genus Murchisonia ; but we are satisfied that the lines of 

 growth do not undergo any inflection or interruption, and that they proceed 

 from behind to the anterior part of the suture at the base of each whorl. 



