PHYLOGENY OF FUSUS AND ITS ALLIES. I3 



FUSUS ACICULATUS Lamarck. 



(Plate I, figs. 3 and 4; connecting form pi. I, fig. 6.) 

 1818. Fiisiis aciculatus Lam'k, Sowerby, Mineral Conchology, vol. III. 



This species is typically developed in the Paris Basin only. In 

 the British Eocene, however, occur individuals which approach this 

 species very closely, and which are generally identified with it. A 

 complete series of specimens may be selected, connecting F. porrcctus 

 with typical F. aciculatus, showing their close genetic relation. As the 

 young F. aciculatus has the characters of F. porrcctus, is is evident 

 that the former is a descendant of the latter. The perfect gradation 

 in characters which is to be expected in a complete series has led 

 some authors to unite the two species. As will be shown later, the 

 Parisian specimens are distinguishable from their British allies bv 

 their more pronounced ornamentation. Whether we call these distinct 

 species, or merely varieties, is of no moment, as long as we recognize 

 the difference between the two and the different degrees of develop- 

 ment each has attained. By retaining the distinct names, we are able 

 readily to demarkate the various steps in the evolution of this particular 

 group. 



A similar series of connecting specimens between the British repre- 

 sentative of Fusus aciculatus and Fnsiis asper may be found, though 

 the characteristics of the latter are always very pronounced. 



The following are the characteristics of the British representatives 

 of this species : 



The protoconch is smooth and obliquely elevated in the first volu- 

 tion, the second having strong smooth vertical riblets. It ends abruptly 

 with a slight varix. 



The conch is characterized in its nepionic stage by the normal 

 round whorl with rounded strong vertical ribs, separated by interspaces 

 which are wider than the ribs. They are ornamented by revolving 

 lines or spirals, which are well marked on the ribs but produce no 

 tubercles or spines. This primitive character continues with scarcely 

 any modifications through the succeeding stages of the conch in F. 

 porrcctus, which thus exemplifies a degree of development in which the 

 adult shell has not passed beyond the normal characteristics of the 

 nepionic stage of the genus. 



In the specimens which lead to the present species, however, a 

 slight modification which is clearly an advance, judging from the 

 normal order of development in more highly differentiated species, 

 occurs in the adult stage. This consists of a slight strengthening of 

 the central spiral, which dimly suggests a carination such as is char- 

 acteristic of most species of the genus. The spirals are commonly 

 uniform and equidistant, except near the upper suture, where new ones 

 make their appearance, which are of course at first thinner and nearer 



