WOOD, PHYLOGENY OF CERTAIN CERITHIIDJE 81 



The original description of this species is as follows: 



Coquille turrieulee, allongeo, a cofes longitiulinales, au nonibre de cinq sur 

 les tours su]ierieurs, niais pins nombreuses sur les autres ; elles sont recoup^es 

 par des sillons transverses au nouibre de trois sur chaque tour. Bouche ovale, 

 anguleuse suporieurement, Ochancrure large, profonde et oblique. 



Horizon and locality : Jurassic, Coral-rag. St. Michiel. France. 



Eemarks : This species of Cerithium shows primitive characteristics 

 in its simple ornamentation consisting of primary spirals and ribs only, 

 its moderate degree of embracing of the whorls and its simple aperture, 

 with short, straight canal. The points of intersection of the spirals and 

 ribs are often elevated, giving the surface a nodose appearance, but the 

 ornamentation is composed essentially of the ribs and three simple spirals 

 only. In this respect the shell is more primitive then C. alhense and 

 must resemble the young of that species before the introduction of inter- 

 calated spirals. 



Specimens of this species were not obtainable, but from the evidence 

 furnished by the literature it seems to be an ancestral form of Cerithium 

 and the earliest representative thus far obtained of the line of evolution 

 terminating in C. tuberosum. 



This species has been referred to the genus Brachytrema. but a com- 

 parison with the type of that genus, B. buvignieri, shows a wide contrast 

 in form and ornamentation, the latter species having the low spire and 

 wide apical angle of the genus Trochus, with the outer lip much ex- 

 panded and the body volution longer than the remainder of the shell. 



V. Summary 



Eeviewing the facts already presented, it is found that, while no speci- 

 mens of true Cerithium were obtained earlier than the Aptien, it appears 

 from the literature that the Jurassic species, Cerithium corallense, may 

 represent the earliest known species of the genus. This is a primitive 

 type of shell of small size, with rounded whorls and having on its adult 

 volutions three spirals crossed by ribs. 



In studying the development of retarded species of Cerithium, it is 

 found that the shell forms first a single spiral, with a second spiral added 

 above the first. Later the two spirals become of equal strength and are 

 crossed by ribs. This stage reappears constantly throughout the genus 

 and its near relatives. The fact that it is so persistent a feature suggests 

 that it probably formed the adult stage of an early Jurassic or Triassic 

 ancestor of Cerithium, from which forms like C. corallense were devel- 



