72 ANNALS NEW YORK ACADEMY OF SCIENCES 



the anterior portion, gives it a sinuous outline. The siphonal canal is 

 broad and deep, bordered by a strong ridge on its columellar side. 



HoBizoN AND localities: Sables Moyens (Upper Eocenic). Widely distrib- 

 uted iu the Paris Basin. 



No. 20148, Columbia University collection. 



Eemarks : The earliest volutions of this shell are missing, but the 

 youngest preserved has an ornamentation similar to that of the ninth to 

 the thirteenth volutions of Potamidopsis tricarinata. It adds an inter- 

 calated spiral as in the latter species, but it diverges from the line of evo- 

 lution represented by P. tricarinata by forming its shoulder angle at the 

 upper instead of the lower margin of the whorl. This species may be 

 regarded as derived from the same ancestral stock as P. tricarinata, but 

 it represents a divergent line of evolution. 



M. Cossmann refers this species to Serratocerithium, a sub-genus 

 founded mainly on characters of the aperture, but the aperture of P. 

 tuberculosa does not appear to differ to a marked degree from that of 

 P. tricarinata. The canal is slightly longer and the outer lip thinner 

 and less flaring. The present reference of the species, being founded on 

 descent as revealed by the ontogeny, necessarily arrives at a different 

 result from one founded on adult characters alone. 



Potamidopsis roissyi Deshayes 



1824. Cerithimn roissyi Deshayes, Desc. des coquilles foss. des environs de 



Paris, II, 322, pi. 50, figs. 13-20. 

 1866. Cerithium roissyi Deshayes, Desc. des anim. sans vert, decouverts dans 



le bassin de Paris, p. 127. 

 1906. Tympanotonus i-oissyi Cossmann, Essais de Paleoconch. Comp., VII, 120. 



Measurements : Length, 25.7 mm. ; greatest diameter, 8 mm. ; apical angle, 

 20° ; sutural angle, 90°. 



The protoconch is missing from the best specimen available, and it is 

 probable that two or three more volutions are also broken away. The 

 youngest volution present is .7 mm. in diameter, and it has exactly the 

 ornamentation of P. tricarinata from the ninth to the thirteenth volu- 

 tions. The same ornamentation continues for four volutions, after 

 which the ribs disappear and the two rows of nodes become equal in 

 strength and often alternate in position as compared across the whorl. 

 On the sixth volution a fine spiral is intercalated at about the middle of 

 the volution. This spiral soon breaks up into a row of extremely fine 

 nodes. At about the same stage of growth the sub-sutural row of nodes 

 becomes stronger than the others, a tendency which increases until the 



