182 CERION. 



It is impossible to separate the short from the long-toothed species 

 on account of the variability of this character among species other- 

 wise closely allied. Occasional specimens G. ylans have the tooth as 

 long as in the type of Strophiops, although as a general rule it is short 

 in that species. Mr. Maynard has proposed several sectional names 

 in this group, based upon the sculpture, etc., of the shells. These 

 are natural groups, though of no great systematic value. They seem 

 more distinct when only the type species is considered than when the 

 entire series of related forms is taken into account. 



The parietal tooth or lamella is long in all of the species of Little 

 Cayman and Cayman Brae ; in C. longidens, hyperlissum and occa- 

 sional specimens of C. maritimum, of Cuba ; and in various species of 

 the groups of C. regina, G. gubernatorial, etc. A division into long- 

 toothed forms (Strophiops) and short-toothed (Maynardia), as pro- 

 posed by Dall, would involve not only the separation of species other- 

 wise closely related, but would cause embarrassment in the grouping 

 of species with the parietal tooth of medium length, or long in some, 

 short in other specimens. The subgenera proposed by Maynard 

 prove to be impracticable when the whole series of species come to 

 be classified, however distinct they may appear in the type species. 

 I have, however, quoted them under the groups in which their types 

 fall, though the limits Maynard would give them are quite unknown. 



The shell is sculptured with fine, close rib-stria3 in the groups of 

 C. crassilabris, C. cyclostomum and G. martensi, and in some species 

 of the groups of C. dimidiatum and C. glans. There is of course 

 every possible transition between the finest and coarsest sculpture, 

 even among species of a single natural group. 



Smooth species occur in the groups of G. uva, pannosum, dimidi- 

 atum, regina, glans, martensi, anodonta and striatellum, and may not 

 unlikely be found in others. In some species, especially of the 

 dimidiatum and martensi groups, the shell may be either ribbed or 

 smooth. 



Spiral striae are developed normally only in the group of C. scalar- 

 inum and some species of the group of G. dimidiatum, 



The inter-relations of the several groups of species annot be ex- 

 pressed in a linear arrangement. The following diagram shows their 

 approximate affinities, on the supposition that G. maritimum repre- 

 sents a less differentiated form, not remote from the ancestral stock 

 of Strophiops ; each group of species being here represented by the 

 name of a typical form. 



