WOOD, PHYLOGENY OF CERTAIN CERITHIIDJE 77 



latter species. From this stage it progresses toward simplification of sur- 

 face forms rather than complication of them, as in the species previously 

 described. It illustrates the fact that progressive development does not 

 always mean complication of structures, but may also travel in the direc- 

 tion of simplification of structures. Although P. trochleare does not 

 add new types of ornamentation, it does emphasize strongly the one 

 feature retained in the extreme prominence of its two continuous spirals. 



Illustrations of progressive development resulting in more simple struc- 

 tures are found in other genera ; for example, Claviger matoni Gray has 

 young stages in which the whorls are ornamented by four strong spirals 

 crossed by numerous oblique ribs, changing abruptly to an ornamentation 

 of two very prominent contin^^ous spirals, but without other features to 

 indicate gerontism as a cause for the loss of ornamentation. 



Deshayes's figures of this species, cited above, indicate an extreme de- 

 gree of variation, including forms with three spirals, or one spiral, or 

 rows of strong nodes, and it is probable that several of these should be 

 considered distinct species. In the absence of specimens showing these 

 variations, they will not be considered here. 



5. Vulgocerithium 

 Vulgocerithium minutum de Scrres 



1822. Cerithium minutum de Serres, Essai pour servir a I'histoire des anim. 



du midi de la France, p. 60. 

 1856. Cerithium minutum Hornes and Partsch, Abhand. der k. k. geol. Reich- 



sanst, III, 390, pi. 41. figs. 8, 9. 



Measurements : Length, 25 mm. ; greatest diameter, 9 mm. ; apical angle, 

 33.5'°, changing to 16° on the last four volutions ; sutural angle, 83.5°. 



The youngest volution preserved on the specimen studied is 1.8 mm. in 

 diameter. It has a well-defined shoulder with two strong spirals, one 

 finer intercalated spiral and four on the shoulder. On the next two 

 volutions the sub-sutural spiral becomes broad, another spiral is added 

 between the two primaries, and one appears on the lower slope of the 

 whorl. On the succeeding volutions the sub-sutural spirals become ele- 

 vated at intervals as a row of nodes, the lower slope of the volution be- 

 comes nearly continuous with its vertical sides and numerous intercalated 

 spirals appear. The adult ornamentation is that of a sub-sutural row of 

 nodes, a median row with nodes slightly more prominent than those of 

 the upper row and ribs which are more or less continuous toward the 

 upper suture but are represented by irregularly spaced nodes below the 



