MAZATLAN UNIVALVES 313 
of the genus.* Clark however speaks of three or four epochs 
of growth in C. trachea, and Stimpson of two (with one inter- 
mediate) in C. pulehellum. The large number of specimens that 
were fortunately disentombed from the worm-eaten galleries of 
Spondyli and the crevices of Chamee and Ostrez, lead to the 
conclusion that some species at least form many successive 
portions ; so that if the whole shell could remain entire, an 
object would be seen resembling an incurved Toxoceras, with 
a Skeneoid apex. Among the 700 Mazatlan Ceca of various 
ages, only one specimen with the spiral portion was found ; and 
not one of the spire alone. (The minute Vitrinelle are perfectly 
distinct.) They are probably so frail as rapidly to perish. 
After repeated examinations of large numbers of individuals, it 
is more easy to say what does not, than what does hold good as 
a specific character. The shell, at different periods of its growth, 
assumes very variable proportions of length and breadth, 
larger or smaller arcs of circles with changeable radu, different 
forms of mouth, greater or less protrusion of apical plug, and 
perhaps opposite styles of sculpture. The different conditions 
are proved to belong to the same species by our continually 
finding shells with the anterior and posterior portions belong: 
ing to different types. Shells in this state were described by 
Prof. Adams as C. monstrosum, and must have been very 
puzzling to an author who in so variable a genus described 5 out 
of 8 species from 8 specimens. The number and disposition of 
the rings, on which several species are founded, is a very vari- 
able character. Perhaps the most constant is the form (not 
the amount of protrusion) of the apical plug; which Prof. 
Adams, with less than his usual minuteness of description, un- 
fortunately passed over, although Searles Wood in his Crag 
Mollusca had called attention to its importance. <A careful 
examination however of the types of 6 out of the 8 species 
described from Panama, which are fortunately preserved in 
the Cumingian collection, has supplied the deficient informa- 
tiou. It seems ungracious, while now describing 16 new 
species from Mazatlan, (the operecula being known in nine) to 
group together 5 out of the 8 already described from the same 
coast. Ihave only done so, because the necessities of the shells 
seemed to require it; and it would have been easy, on the 
principles followed by, Prof. Adams, to have increased the 
number of Mazatlan species four-fold. The plan here adopted 
~ - ‘septum, marking the point at which the original spire has been 
cast off.” Forbes 4 Hani, loc. cit. p. 176, 
Aug. 1856, dd 
