WOOD, PHYLOGENY OF CERTAIN CERITHIID/E 71 



species in the genus Ptychopotamides is indistinct, and does not consti- 

 tute a sufficient reason for separating the species from the group to which 

 it is closely related in its development. 



Potamidopsis tuberculosa Lamarck 



1804. Cerithium tuherculosum Lamarck, Ann. du Mus. Nat. d'hist naturelle, 



III, 348. 

 1824. Cerithium tuberculosnm Deshayes, Desc. des coquilles foss. des environs 



de Paris, II, 48, figs. 1-5. 

 1866. Cerithium tuherculosum Deshayes, Desc. des animaux sans vert. dScou- 



verts dans le bassin de Paris, III, 122. 

 1906. Serratocerithium tuberculosum Cossmann, Essais de Pal6oconch. Comp., 



VII, 75. 



Measurements : Length, 33 mm. ; greatest diameter, 12.2 mm. ; apical angle, 

 26° ; sutural angle, 86.4°. 



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

 of fifteen volutions. The protoconch of the specimen is missing, and the 

 first two volutions are too much worn to show the surface ornamentation, 

 but the third volution present is ornamented by a sub-sutural spiral and 

 a stronger spiral forming a shoulder angle just above the suture. Both 

 spirals are rendered nodose and connected by ribs which are continuous 

 across the volutions. This type of ornamentation persists for five volu- 

 tions and is closely Similar to the ornamentation of P. tricarinata at a 

 slightly later stage. The nodes of the lower and upper rows gradually 

 become equal in strength, and the shoulder disappears. On the sixth 

 volution of those preserved a spiral is intercalated between the two rows 

 of nodes already formed, and two volutions later this spiral is broken up 

 into a row of very fine nodes. The ornamentation at this stage resem- 

 bles that of P. tricarinata at the same stage, except that the lower row 

 of nodes is more prominent on the latter species. At the eleventh volu- 

 tion the nodes of the sub-sutural row become more prominent and more 

 widely spaced than those of the lowest row. This tendency increases 

 until a shoulder angle composed of strong, transversely elongate nodes 

 is formed just below the suture. This becomes the ornamentation of the 

 adult shell, except that a fourth spiral which is partially exposed just 

 above the suture also becomes wavy or faintly nodose. The body volu- 

 tion bears one nodose ridge in addition to those already described and 

 several fine spirals on its lower slope. 



The aperture is nearly circular. The callus of the inner lip is brofid 

 and thin. The outer lip is thin; its margin, growing more rapidly nl 



