MAZATLAN UNIVALVES 321 



Coecum firmatum, C. B. Ad. Pan. Shells, no. 213, pp. 161, 311. 



+ Ccecum eburneum, C. B. Ad. do. no. 212, pp. 161, 311. 



+ (adolesc.) Ccecum monstrosum, C. B. Ad. do. no. 216, pp. 162, 



311. 



+ (adolesc.) Coecum pygmaeum, C. B. Ad. do. no. 218, pp. 163, 311. 

 ? + (jun.) Coecum 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. pygmaeum) which, through 

 the wonted kindness of H. Cumin g, 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. pygmseum. 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 in 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. Hibs 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 long. '067, lat. "006 '012. 

 The next shell, measuring long. '048, lat. '008 '013, is grace- 

 fully bent and tapering, and entirely ringed. The next, in the 

 4 pygmaeum' state, measures long. '068, 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, lai. '016 '025. 



