MAZATLAN UNIVALVES 321 
Ceecum firmatum, C. B. Ad. Pan. Shells, no. 213, pp. 161, 311. 
+ Coecum eburneum, C. B. Ad. do. no. 212, pp. 161, 311. 
+(adolese.) Coecum monstrosum, C. B. Ad. do. no. 216, pp. 162, 
311. 
+(adolese.) Coocum pygmeum, C. B. Ad.do. no. 218, pp. 163, 31] 
?+(jun.) Ceecum diminutum, C. B. Ad. do. no. 211, pp. 161, 310. 
The fortunate preservation of specimens in the Cumingian 
collection, received from Prof. Adams himself, (viz. 3 of C. 
firmatum out of 85, 3 of C. eburneum out of 22, 2 of C. mons- 
trosum out of 7, and 1 of two of C. pygmzeum) which, through 
the wonted kindness of H. Cuming, Esq., I have been enabled 
to subject to a rigid microscopical examination and comparison 
with the Mazatlan specimens, enables me to give the above 
synonymy with tolerable confidence, in spite of the worn state 
of most of the shells. Of C. diminutum, Prof. Adams only 
found one specimen. As far as can be judged from the des- 
cription, it is the early stage, long and slender. The next 
stage, appears to be C. pygmeum. C. monstrosum partakes 
half of this, and half of the adult form. <A similar state is 
found in other species, when there is any quantity of specimens. 
C. eburneum and C. firmatum are to one another as the English 
forms C. imperforatum and trachea. The rings im the Cum- 
ingian specimens of C. eburneum vary from 26—33, and are 
more or less pressed together near the aperture. This char- 
acter is extremely variable. The largest specimen here given 
is exactly intermediate between the two forms. It appears 
very rare at Mazatlan, where it is represented by C. undatum. 
C. firmatum is distinguished from nearly allied species by 
the following characters. Ribs somewhat flattened and squared, 
with deep rather squared interstices. Plug intermediate be- 
tween the ungulate and mucronate forms ; rising gradually in 
a nearly straight line to the summit, which is placed rather to 
the right, somewhat broad but thin, with the apex rounded. 
The youngest shell (probably belonging to this species, but the 
plug is rather imperfect) is quite smooth in the first part, 
but afterwards ringed ; it measures Jong. *067, lat. *O06— O12. 
The next shell, measuring long. ‘048, lat. *OO8—*013, is grace- 
fully bent and tapering, and entirely ringed. The next, in the 
‘pygmeeum’ state, measures long. ‘063, lat.°013—017. The 
rings on first appearing are rather sharp and distant: they 
gradually assume the normal type; sometimes however run- 
ning into each other, in the ‘eburneum’ state. The largest 
specimen measures long. ‘1, lat. ‘016—'025. 
