CIRROBRANCHIATA. MOLLUSCA. DENTALIUM. 155 



Shell small, thin and fragile, elevated, compressed at the sides, 

 so that the margins are nearly parallel, the ends of equal breadth, 

 and obtusely rounded ; apex at the posterior third, pointing for- 

 wards ; outer surface beautifully checked with the lines of growth, 

 and fine, but distinct, radiating lines ; color a reddish-brown, with 

 oval or circular yellowish-white spots, arranged in a somewhat 

 regular manner, so that the whole resembles a net-work. The 

 thinness of the shell allows the external coloring to appear on the 

 inside ; edge entire. Length j\ inch, breadth -j% inch. 



Found abundantly on the eel-grass {Zostera marina)^ to whose 

 narrow leaves its form is exactly adapted. 



In old specimens a lateral compression is very obvious, and the 

 sides are at least parallel, and sometimes incurved for one half the 

 length of the shell. The apex, when not worn off, is acute, and pro- 

 jects distinctly forwards. The markings usually give the shell a 

 checkered appearance ; but occasionally we have stripes, as in the 

 preceding species. 



This shell is the very miniature of Patella compressa. Mr. Sower- 

 by suggests that it bears the same relation to P. testudindlis as Patella 

 compressa does to P. minidta ; in other words, it is the same spe- 

 cies, changed in form from having adhered to a narrow sea-weed 

 instead of a stone. The general marking of the shell, and the cir- 

 cumstance of its seldom, if ever, being noticed living anywhere except 

 upon the naiTow leaves before mentioned, render this opinion not at 

 all improbable. 



Family CIRROBRJUTCHMT^, Blainv. 



Animal with the branchiae in the form of numerous long filaments, arising 

 from two radical lobes placed above the neck, and enveloped, tvith the head, by 

 the mantle. Shell tubular, not spiral. 



Genus DENTALIUM, Lm. 



Shell tubular, elongated-conical., slightly curved, opening at each 

 end by a rounded orifice. 



Dentalium dentale. 



Shell polished.) slightly curved^ icith eighteen or tiventy faint^ 

 unequal ribs. 



