314 MAZATLAN UNIVALVES 
was to take first those species (C. undatum and C. leve) 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 io 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 -~ - - - species, 71 individuals. 
Anellum - - - - - Ge ale “ 
Fartulum~ - = See : Ya ae eer Aly a 
Total | ee RM aeepee 832 5 or 
Section A. ELEPHANTULUM. 
C. t. majore, tereti, solidiore, equaliter arcuatd, apertura vie 
contracta, postea expansd, parum declivi, adultd longitudin- 
aliter sulcatd, testa juniore haud subiter mutante. 
The shells thus grouped are like minute Dentalia Hlephan- 
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. From the appearance of the youngest, it 1s 
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 Griguon 
