niYLOGENY OF FUSUS AND ITS ALLIES. 59 



cancellated by strong spirals. In the succeeding whorls the ribs in- 

 crease in strength, but continue to extend across the entire whorl. 

 The central spiral increases slightly in strength, without producing an 

 angulation. The shoulder also remains rounded. Intercalated spirals 

 appear early, but do not reach the strength- of the primary spirals. In 

 some cases this intercalation becomes compound. Towards the end 

 the ribs either grow stronger, leading to such forms as fig. 5, pi. VII, or 

 else become obsolete, leading to such species as F. semirugosns Bell, 

 et Mich. (PI. VIII, fig. 9-12) and F. cinctus Bell, et Mich. 



In general aspect, and particularly in the character of its ribs, 

 this species is not unlike some varieties of Clavilithes riigosiis, this 

 Eocene species having attained the same degree of development which 

 F. rostratiis reaches in the Pliocene. 



From the round-whorled variety, in which the strengthening of the 

 central spiral alone marks the beginning of a peripheral angulation of 

 the whorls, a branch leads to the strongly angulated forms which are 

 typified by F. bredcc Mich. Even within the limits of the present 

 species angular varieties may be recognized, in which the shoulder 

 becomes somewhat flattened and the central spiral transformed into a 

 keel (see pi. VII, fig. 4). Not infrequently the last whorl returns 

 to the more primitive rounded condition with (usually) swollen ribs, 

 which is probably a gerontic feature. Figure 5, pi. VII, illustrates 

 a specimen in which this feature has been developed to an extreme 

 degree. As far as the penultimate w^horl, this specimen shows all the 

 characters of an angular variety of F. rostratiis. These features come 

 to a sudden stop at a point which evidently marks an old break or 

 injury to the shell. This resulted in the modification of the last whorl 

 and ribs, which have passed into a condition in which they resemble the 

 gerontic features of the last portion of a normal old age individual. 

 The whorl itself is rounded and has strong rounded ribs which are 

 more of the nature of regular folds than of ribs. These folds are 

 separated by wide interspaces and extend from suture to suture. 

 Towards the end they become strongly bulging, especially at the 

 periphery of the whorl, where the last one of the ribs projects about a 

 quarter of an inch. While at first these are true folds in the shell, 

 affecting the interior as well as the exterior, they quickly become 

 solid by the deposition of secondary calcareous material, so as to be no 

 longer visible on the inside of the whorl. The spirals are strongest in 

 the center of the whorl, where three of them are especially prominent. 

 They decrease in prominence towards the sutures. Intercalation of 

 spirals is compound. 



This specimen shows that premature gerontism may be induced 

 by injury, the resulting growth being similar in all respects to the 

 normal gerontic growth in an old individual. 



