82 A'SfNALS XEW YORK ACADEMY OF SCIENCES 



oped by the addition of a third spiral above the two primary ones. Such 

 a type of development is epitomized in three species of the upper Eocenic. 

 C. bicarinatum has adult ornamentation like the supposed primitive an- 

 cestor of C. coraUense — that is, two equal spirals crossed by ribs (plate 

 iv^ fig. 9). The immediate descendant of the former species is C. bicari- 

 natum mut. trispirale, a form which corresponds with C. coraUense in 

 having a third spiral present above the two primar}^ ones, producing a 

 shell with rounded volutions and an ornamentation of three simple spirals 

 crossed by ribs. 



The lower Cretacic species, C. albense, shows an advance upon C. 

 coraUense in the introduction of intercalated spirals, and it has its par- 

 allel in the Eocenic species, C. retardatum. which differs from its imme- 

 diate ancestor, C. bicarinatum mut. trispirale, only in the presence of 

 intercalated spirals. C. bicarinatum is descended from an early ancestor 

 of Cerithium, and is so retarded as to retain its ancestral characteristics 

 nearly unchanged. Its descendants pass through rapidly, in one geo- 

 logical period, a path of evolution which has been traveled more slowly 

 in the main line of evolution from early Jurassic, or possibly Triassic, to 

 Cretacic time. 



An advance upon the type of development shown in C. albense is seen 

 in C. cornuelianum, of later Cretacic (Aptien) time. This shell has 

 many intercalated spirals, and two of the primary spirals are stronger 

 than the others, forming a slight projection around the median portion 

 of the adult volutions. This projection, although similar in form, is not 

 the developmental equivalent of the primary two-spiraled stage, for in 

 the phylogeny of the genus it appears after the stage with three primary 

 spirals and after the development of intercalated spirals. In recent, 

 highly accelerated species of the genus, intercalated spirals appear so 

 early in the ontogeny that the stage with three simple spirals is either 

 omitted altogether or obscured by the fact that the third spiral on the 

 shoulder never becomes as strong as the others. In such cases the stage 

 just described seems to be continuous with the primitive two-spiraled 

 stage, from which it differs morphologically only in the presence of in- 

 tercalated spirals. 



The greater abundance of Eocenic material in the collections studied 

 furnishes an opportunity for determining the phylogeny with greater 

 certainty than in the earlier horizons. At that time C. cpquispirale rep- 

 resents the next stage in the phylogeny of the genus beyond C. cornue- 

 lianum, for on this specimen the two equal spirals are well developed and 

 persist to the adult whorls, and spirals of several orders are easily recog- 

 nizable. The young stages of C. cpquispirale are unfortunately missing, 



