76 ANNALS NEW YORK ACADEMY OF SCIENCES 



lasts for a variable number of volutions on different specimens, but 

 usually from five to eight volutions. Beyond this the ribs disappear, 

 and the shell is ornamented for the remainder of its growth by two ex- 

 tremely prominent continuous spirals with deep concave depressions 

 between them. A third spiral, partly concealed by the embracing of the 

 whorls, is shown on the body volution to be less prominent than the 

 others. 



The aperture is distinctly quadrangular. The callus of the inner lip 

 is thick and rather narrow. The siphonal canal is short and deep. The 

 outer lip is often thickened in large individuals, sinuous and folded into 

 lobes to correspond with the spirals of the outer surface. 



Horizon and localities : Oligoceuic. Moriguy, Jeures and inauy other local- 

 ities in the Paris Basin. 



No. 20152, Columbia University collection. 



Eemarks : The youngest volution of this species to show the surface 

 has the ornamentation of the adult P. tricarinata. The first change to 

 take place in this ornamentation is in the loss of the median spiral, which 

 was the last feature to be acquired in the development of the latter spe- 

 cies. The succeeding three or four volutions of P. trochleare correspond 

 with a still younger stage of P. tricarinata, namely, that with two rows 

 of nodes only. The adult P. trochleare has two continuous spirals, as 

 in the earliest stages of P. tricarinata. The facts thus far stated seem 

 to indicate a loss of characters by gerontism and the return to the primi- 

 tive conditions of an ancestor, but that this is not the case is shown by a 

 study of the intervening stages of P. tricarinata. Studying the develop- 

 ment of that species in reverse order, the stage preceding that with two 

 rows of nodes only is one in which the sub-sutural row of nodes is absent, 

 and, earlier still, a fine spiral is present immediately above the shoulder 

 spiral (plate viii, fig. 7). This is not the same feature as the median 

 row of nodes in the adult, although it occupies the same position, because 

 it disappears and gives place to another character before the appearance 

 of the adult median nodes. It is this spiral, however, which, on the 

 fourth volution, is as strong as the spiral at the shoulder angle and forms 

 the upper of the two continuous spirals on the second volution. In P. 

 trochleare, on the contrary, it is the sub-sutural spiral which persists and 

 forms the upper of the two strong spirals in the adult. The lower of the 

 two corresponds with the spiral at the shoulder angle in P. tricarinata. 

 These two spirals have not the character of primitive structures, but are 

 on the contrary, extreme in their development. P. trochleare is a de- 

 scendant of P. tricarinaia. with its youngest stages like the adult of the 



