258 BULLETIN OF THE 



whose family it belongs. I have been somewhat surprised to find that all the 

 specimens in the National Museum belonging to this species come from be- 

 tween Cape Cod and Sandy Hook. All the specimens marked B. nigrum from 

 south of the harbor of New York, beginning with those from Crisfield, Md., 

 are of another species, B. varium Pfr. The latter, rather small at the north, 

 southward grows larger as the climate becomes more congenial, and its variety 

 pallidum of Pfeiffer includes several later synonyms. It has been rather widely 

 distributed as B. gibherulum C. B. Adams, and belongs to the section Diastoma 

 of Deshayes.* Closely related to these shallow-notched Bittiums are the 

 Rissoa tervaricosa and (its variety) melanura of C. B. Adams, which have been 

 referred to Alaba. Whether congeneric with the original Alaba A. Adams, 

 I have not the material for determining; but they differ from Bittium in 

 having the notch nearly or quite obsolete, more delicate sculpture, and the 

 nucleus more slender and with more whorls. A, tervaricosa C. B. Adams is 

 found in South Florida and the Antilles. A typical specimen of ^. melanura 

 differs only in having the nucleus dark colored, a variable character. 



A. Adamsi Dall is a smaller species, opaque white with usually a brown tint 

 on the early whorls; an anteriorly rounded aperture; about nine rounded 

 whorls ; an acute nucleus and subreticulate sculpture, the spirals being stronger 

 than the transverse waves or threads. It is found from Hatteras southward; it 

 has few varices, an imperforate base, and not a trace of a notch or truncation 

 anteriorly. A still smaller form, A. cerithidioides Dall, is distributed through 

 about the same range. It is much more elegant in appearance than the others, 

 though only three millimeters long and one and a quarter in diameter. The 

 shell has an acute nucleus of three and a body of seven whorls, which are trans- 

 lucent with delicate brown painting. The nucleus is pale and glassy; the early 

 whorls have one or two well marked peripheral threads, the later ones have 

 spiral threads in front of the periphery and on the base. The transverse sculp- 

 ture which crosses the whorls is composed of fine regular even elegantly con- 

 cavely arched riblets, to the number of at least thirty ; these become obsolete 

 on the rounded base, and there is a minute chink or even a perforation behind 

 the pillar, and a patulous suggestion of a notch at the anterior margin of the 

 aperture. The shell suggests a minute Cerithidea which has not begun to form 

 its lip, and is one of the most elegant of our minute species. It has only one 

 faint varix. 



The genus Mathilda, which might be placed in a variety of situations sys- 

 tematically, and which is to some extent recalled by these little shells, I have 

 placed provisionally next to the Turrilellidce. 



A list of the species of Cerithium, positively identified as from this region, 

 from the collection of the National Museum, may be useful, omitting reference 

 to subdivisions and most of the synonyms. 



We have our largest and most distinct American species in the C. Jloridanum 



* Diastoma was considered a Rissoid by Deshayes. I cannot doubt its pertinence 

 to this group of Bittium, which can hardly be separated by the shell from Alaba. 



