32 ANNALS NEW YORK ACADEMY OF SCIENCES 



Horizon and locality : Recent. Islaud of Guadeloupe. 

 No. 20136, Columbia University collection. 



The shells of the Cerithium tuherosum group, although differing greatly 

 in size, have a general similarity in form and in the essential character- 

 istics of the aperture. Their variation is expressed most strongly in the 

 features of the surface ornamentation. These features consist of spirals 

 of the first and higher orders, ribs, nodes which may develop into spines, 

 and a shoulder varying in extent of development and in form. In order 

 to bring out more clearly the relation between these shells as expressed in 

 their surface ornamentation, they have been plotted in diagrammatic 

 form, arbitrary signs being chosen to represent certain features of the 

 ornamentation. On plate i the numbers at the left of the page indicate 

 the volutions, the protoconch being numbered one. The protoconch, 

 when present, is represented by a circle. The spirals are represented as 

 horizontal lines, for while they are actually longitudinal elements, on the 

 shell they appear horizontal, and the diagram being intended merely to 

 represent certain features in graphic form, it retains the line most readily 

 associated in the mind with what is seen on the shell. For a similar 

 reason ribs are represented as vertical lines. Secondary spirals and those 

 of higher order are represented by two horizontal lines. Ixodes are indi- 

 cated by dots, and the outline of the volution is represented by the lines 

 which would be used in drawing its two upper slopes — that is, an obtuse 

 angle changing to a right angle to agree with the development of certain 

 shells. The diagram is intended to represent resemblances and differ- 

 ences in a general way only. It records the introduction of spirals, ribs 

 and nodes, but does not attempt to show the different kinds and degrees 

 of development which they attain or such features as size, degree of em- 

 bracing of the whorls, etcetera. More complete details are given in the 

 descriptions and figures of the different species. 



The diagram brings out the fact that some species are more retarded 

 than others; for example, CerifJmim menl-ei is a more retarded shell than 

 C. adansoni, as shown by the fact that on the former species the change 

 in the outline of the whorl does not take place until the ninth volution, 

 and nodes are not acquired until the last volution, while both these 

 changes occur much earlier on C. adansoni. A similar comparison has 

 already been made between 0. tuberosum and C. adansoni. 



The diagram also shows differential acceleration and retardation of re- 

 lated species as in C. echinatum, and C. graciliforme. On the former the 

 oblique-angled outline is lost on the seventh volution, while retained until 

 the tenth on the latter. ISTodes are, however, acquired on the ninth volu- 

 tion of C. graciliforme. and do not appear until the twelfth volution of 



