PHYLOGENY OF FUSUS AND ITS ALLIES. 47 



correspond to the weaker of the two. Thus a symmetrical arrange- 

 ment of spirals is produced, the central one being the strongest, while 

 on either side occurs a weaker one. From this the spirals decrease in 

 strength towards both sutures. Intercalation becomes compound and 

 the interior of the labrum becomes strongly lyrate. 



A number of specimens of this variety show a tendency towards 

 obsolescence of the tubercles on the last part of the last whorl. These 

 lead to the keeled variety. 



In the collection of the Philadelphia Academy of Natural Sciences 

 occurs a specimen which is unusually slender, but otherwise has all the 

 characteristics of this variety. The early whorls agree precisely with 

 those of typical individuals, but in the later whorls the shoulder is 

 much more inclined. The ribs are obsolete on the last three whorls, 

 which are characterized only by tubercles. This variety is to var. 

 nodosus what F. meyeri is to the typical form of the species. 



The Typical Variety. 

 (Plate V, figs. 2 and 3.) 



This variety is the coins type of the series, the tubercles having dis- 

 appeared, while the central carina continues strong. We have here an 

 acceleration, the earlier stages being condensed. The angular noduled 

 whorls {torcinnus stage) pass gradually into the smoothly keeled whorls 

 {coins stage), the occurrence of both stages on this shell constituting 

 the coins type. The keel appears to be produced through a confluence 

 of the nodes, which become elongate and flattened. 



The spirals often become highly compound and the various lines 

 being closely crowded, a broad aspect is given to the spirals. 



This variety is connected by a longicaudiis-toreiimoidcs type with 

 the longicaudiis type (see PI. V, where fig. 4 represents the former 

 and fig. 5 the latter). In fig. 4 the angular whorls precede the round 

 whorls, thus forming a connection between figs. 3 and 5. 



Variety APLICATUS var. nov. 

 (Plate V, fig. 5.) 

 This variety represents the longicaudiis type of this series. The 

 carinated {torcnmns) stage is wanting, this being a case of acceleration 

 by elimination, as compared with the preceding variety which repre- 

 sented acceleration by condensation. In any normal series the latter 

 always precedes the former. A slight flattening of the shoulder and 

 the strong development of the two central spirals give the early whorls 

 a subanghlar appearance, but the ribs continue uniformly across this 

 angulation. The torcinnus stage may then be considered dropped out 

 in this variety, the coins stage succeeding the turricnlns, and being suc- 

 ceeded by the longicaudiis stage. 



