12 PHYLOGENY OF FUSUS AND ITS ALLIES. 



primitive condition yet obtained in the conch of a true Fiisiis. In the 

 succeeding stages of development, the shells of this species are scarcely 

 modified. Several additional spirals appear between the upper suture 

 and the spiral next to it. One of the central spirals is slightly 



strengthened in the adult stage is some speci- 

 mens, suggesting a central carination. This 

 incipient carination is scarcely noticeable in 

 the majority of specimens which are referable 

 to this species. The interspaces between this 

 central spiral and that on either side of it 

 are broader than those between other spirals. 

 On the body-whorl of a large specimen (PI. 

 I, fig. ii) there are thres uniform and 

 Fig. I. F. porrcctus, equidistant spirals on either side of the cen- 

 showing protoconch and tral one. Above the upper set of three 

 early conch whorls. En- spirals, next to the suture, two additional new 

 . .(,!••• spirals have appeared. Below the three 



lower spirals occurs a broad interspace, and 

 below that the spirals of the spindle. These are very oblique, diverge 

 strongly and are early supplemented by secondary spirals, intercalated 

 between the primary ones. In the preceding whorls, the first or 

 uppermost of the spirals of the spindle appears just below the suture 

 of the overlying whorls. The costse become flattened on top in the 

 adult. They are strong and slightly narrower than the interspaces 

 separating them. The strengthened median spiral is not always in 

 the center of the whorl. Frequently it is somewhat above the center, 

 indicating that increase in breadth of whorl with continued growth 

 was less on the suture side than on the side of the spindle. 



In accelerated individuals of this species intercalated spirals appear 

 occasionally. These arise in the broader interspace above the spirals 

 of the spindle, and thus appear on the lower part of the body-whorl. 

 A specimen from Hampshire, 62 mm. long, has the intercalated spirals 

 in the penultimate whorl. 



In some of the specimens from Hampshire, there are only two 

 spirals on the body of the whorl, below the central carinated one; 

 a condition eminently characteristic of F. aspcr Sowerby. These 

 may represent transitional forms. 



Localities: Barton Cliff (M. C. Z. 1402) ; Hordle Cliff, London 

 clay (M. C. Z. 1400) ; Barton Beds, Hampshire (Stud. Pal. Coll. Harv. 

 Univ. 120). 



Horizon: London Clay, Barton Beds. 



This species is among the most primitive of the genus yet discov- 

 ered. It retains the normal nepionic characters of the genus through- 

 out life with scarcely anv modifications. 



