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. pulchellum. The large number of specimens that 

 were fortunately disentombed from the worm-eaten galleries of 

 Spondyli and the crevices of Chamse and Ostrese, 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 Cseca of various 

 ages, only one specimen with the spiral portion was found ; and 

 not one of the spire alone. (The minute Vitrinellse 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 radii, 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 Seaiies 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- 

 tion. It seems ungracious, while now describing 16 new 

 species from Mazatlan, (the opercula being known in nine) to 

 group together 5 out of the 8 already described from the same 

 coast. I have 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 $ Hani. loc. cit. p. 176. 



Aug. 1856. dd 



