364 MAZATLAN UNIVALVES 



GENUS TKUNCATELLA, Eisso. 



423. ?TEUNCATELLA -- , sp. ind. 



Tablet 1721 contains an extremely young shell and an older 

 fragment, which may belong to this genus. The young shell 

 looks like an Aclis ; but the vertex, as in Tr. Montagui is not 

 sinistral, but mammillated, with the first whirl sunken and 

 scarcely oblique. It has three and a half normal and very 

 tumid whirls, scarcely diverging ; and is very much smaller 

 than the corresponding state of Tr. Montagui, only measuring 

 long. *043, lat. *015. The fragment is '027 across, with a con- 

 tinuous peritreme, large umbilicus and smooth exterior. 

 Hob. Mazatlan ; off Spondylus, extremely rare ; IS pool Col. 



FAMILY 

 GENUS PLANAXIS, Lam. 



4- 424. PLANAXIS NIGEITELLA, Forbes. 



Proc. Zool. Soc. Dec, 1850, p. 273, pi. 11, f. 6. 



= Planaxis acutus, Mice, in Zeit. f. Mai. Kov. 1850, p. 169, 



no. 23. 

 4- Planaxis obsoletus, MJce. loc. cit. p. 170, no. 24. 



Although Menke's names have a month's priority, yet as the 

 description of Forbes is much more accurate, and represents 

 the whole species, while Menke only indicates the extreme 

 forms, it is here preserved. It is given on the authority of the 

 Kellettian collections as from San Juan Del Fuaco :. but as it 

 is an extremely abundant shell, and yet was not found, either 

 there or along the Californian coast, either by Mr. Nuttall or 

 the Transatlantic collectors, it is probable that the specimens 

 were collected at San Juan in the Gulf of California, or even 

 at Mazatlan. That they are identical with the Mazatlan species 

 is proved on comparison by the remarkable epidermis, which 

 lies in an irregular mass, grooved in a comb-like - manner 

 diagonally ; and when this is removed, there appears a fine 

 velvety coat lying in the interstices. This peculiarity, by 

 which it may easily be separated from the very similar P. semi- 

 sulcata of the W. Indies, is found in both of Menke's forms, 

 which pass into each other by insensible gradations. The 

 nucleus has three tuberose whirls, with a flattened apex, very 



