WOOD, PHYLOGEXY OF CERTAIN CERITHIIDJE 55 



The early stages of this species are described from a young individual 

 of ten volutions. The protoconch of this specimen is missing, but the 

 youngest volution preserved is .5 mm. in diameter and is probably the 

 first volution beyond the protoconch. It is ornamented by a single 

 spiral only, and no ribs arc present. Another spiral is added above the 

 first on the next volution, and on the third volution of those preserved, 

 the two spirals become of nearly equal strength. Eibs first appear on 

 the fourth volution and on the fifth, a fine third spiral appears above the 

 two primaries. The three spirals are well developed on the seventh volu- 

 tion, and on the eighth the first intercalated spiral is introduced between 

 the two lower primary ones. Another intercalated spiral soon appears 

 between the two upper primary spirals, and the adult whorls have one 

 spiral intercalated between each pair of primaries, with two fine spirals 

 above and two below the three primary spirals. The body volution has 

 two strong and six very fine, closely set spirals on its lower slope. 



The aperture is similar to that of C. hicarinatum, being nearly circular, 

 with shallow anterior canal. The callus of the inner lip is wider and 

 the posterior ridge better developed than on C. hicarinatum. The outer 

 lip is somewhat broken, but the lines of growth indicate that its lower 

 margin grew more rapidly than the upper one. 



Horizon and localities: Sables Moyens (Upper Eocenic). Le Gu^pelle, Acy- 

 en-Multien, Paris Basin. 



No. 20146, Columbia University collection. 



Remarks : Cerithium retardatum represents a step in advance of C. 

 hicarinatum mut. trispirale in the development of intercalated spirals. 

 The three forms, C. hicarinatum, its mutation, trispirale, and C. retarda- 

 tum, constitute a direct series in evolution. C. hicarinatum develops 

 ribs and two equal primary spirals only. The mutation trispirale car- 

 ries the development farther in the growth of a third primary spiral, and 

 C. retardatum advances still farther in the introduction of intercalated 

 spirals. 



C. retardatum illustrates the law of recapitulation, for its fifth volu- 

 tion is like the adult of C hicarinatum, and its seventh volution repre- 

 sents the adult of the mutation trispirale. At the same time the law of 

 acceleration is illustrated by this shell, since the evolution represented by 

 the entire life history of both C. hicarinatum and C. hicarinatum mut. 

 trispirale is passed over in the first seven volutions of C. retardatum. 

 But while C. retardatum is accelerated as compared with C. hicarinatum, 

 it is retarded as compared with ('. a'cjiiis/)irale and other sliells occurring 

 in the Upper Eocenic. It may be compared with the Upper Cretaeic 

 C alhense in the main line of evolution of the C. tuherosum group. 



