M. D'Archiac on the Genus Murchisonia. 



281 



striae of growth on leaving the suture of the whorls are arched 

 anteriorly, and then take a backward direction as far as the 

 keel or fillet. On these they form a curve convex posteriorly, 

 and beyond the striae take a forward direction, again curving 

 as they approach the base of the aperture. It is chiefly the 

 constancy, and especially the perfect regularity of the fillet 

 or keel, throughout the whole length of the spire, which has 

 determined us to separate these shells from Cerithium ; for in 

 the latter genus we find species such as Cerithium lineola, 

 echino'ides, involutum, Cordieri, acutum, variabile, turris, &c, 

 having a sinus on the right lip, which is always indicated on 

 the whorls by the inflections of the striae of growth. The 

 slight curvature of the columella and the shortness of the 

 canal, in most species from the secondary formations, appear 

 to point out the affinity of Murchisonia rather to the Pota- 

 mides than the true Cerithince. The Potamides do not ap- 

 pear to us to be more exclusively fluviatile than the Cerithince 

 themselves to be exclusively tertiary or recent. We may 

 lastly remark, that the Murchisonice represent the Cerithince 

 and Turritellce in the ancient formations, as the fossil Pleuro- 

 tomarice represent the existing Trochi. 



The following is our definition of the genus Murchisonia : 

 Shell turrited ; aperture oblong, oblique, terminated at the 

 base by a very short or truncated canal. Columella 

 curved, slightly recurved outwardly. A notch, more or 

 less deep, on the right lip, narrow, the edges parallel, 

 the successive closing of which produces on the middle 

 of the whorls a simple or double keel, or rather a con- 

 tinuous fillet or band, well defined throughout the whole 

 length of the spire. 



seems probable that the mantle of the animal was also deeply notched ; but 

 from the two lips or edges of the notch being in contact, instead of a greater 

 or less fillet or keel, there are two calcareous plates superimposed, and the 

 striae of growth, which on each of them diverge in opposite directions, in- 

 dicate that they are formed by two sepai*ate portions of the mantle. In 

 confirmation of our view of the manner in which the closing of the notch 

 takes place in these three genera, we generally find that the wider the fillet 

 the less salient it is. In fact, it is obvious that the two lips of the notch in 

 the mantle being wide asunder, they cannot form either a fillet or keel ; 

 the notch would then only be closed at its posterior extremity, independ- 

 ently of the two portions of the mantle, which would secrete above and 

 beneath the right lip of the aperture. On the contrary, when they approach 

 and secrete calcareous matter at the point of contact, the two lips of the 

 notch would form a keel, elevated and marked in proportion as their con- 

 tact was more intimate. In certain shells, as in Schizostoma radiata, the 

 plates thus produced are completely united. The importance, as a specific 

 character, of the greater or less width or elevation of the fillet or keel does 

 not appear to us to be considerable ; and we shall show elsewhere that the 

 proportions of the notch are very variable in the same species. 



Ann. 2$ May. N. Hist. Vol.ix. U 



