314 MAZATLAN * UNIVALVES 



was to take first those species (C. undatum and C. laeve) of 

 which, there were the greatest number of specimens presenting 

 well marked characters ; to ascertain the mode of growth and 

 power of variation in these ; then to apply principles so arrived 

 at to the elucidation of the rarer forms. 



To save labour in the identification of species, they are here 

 grouped under 3 sections, according lo form and sculpture. 

 The plug also is described as either (1) mammillate, when it is 

 uniformly convex, without apex, as in C. glabrum ; (2) ungu- 

 late, when the whole surface gradually rises towards the back, 

 where it ends in a long but narrow apex, as in C. trachea, 

 presenting in profile the appearance of a hoof projecting from 

 the ankle of the shell ; or (3) mucronate, when it is nearly 

 flat over the principal part of the surface, rising suddenly into 

 a knob at the back. The projecting profile is described as 

 "margo lateralis." 



The largest and finest shells belong to the group Elephantu- 

 lum; this is however scarce in individuals, and the typical 

 condition of the genus appears to be in the Anellum group.* 

 Their relative proportion of individuals and species, as found 

 in the Mazatlan collection, may be thus summed up. 



Elephantulum .... 5 species, 71 individuals. 



Anellum 6 412 



Partulum 7 210 



Total 18 693+ 



SECTION A. ELEPHANTULUM. 



C. t. majore, tereti, solidiore, cegualiter arcuatd, cvpertura vix 

 contractd, posted expansd, pa/rum declivi, adultd longitudin- 

 aliter sulcatd, testa juniore haud subiter mutante. 



The shells thus grouped are like minute Dentalia Elephan- 

 tina. They differ from the more common forms in their greater 

 size and proportionate length, and in the regularity of the 

 outline, both when young and adult. They appear to be rare, 

 both at Mazatlan and in other seas. Very few young speci- 

 mens were found. Prom the appearance of the youngest, it is 

 probable that these, like the annulate species, are smooth in 

 the earliest stages. The number of ribs appears to be a very 



* Of a fourth group, Inflatulum, only one species is yet known, recent in 

 sponge, fossil at Grignon 



