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LETTER VIII. 
NEARLY STRAIGHT OR IRREGULARLY TWISTED SHELLS, WITH SIMPLE 
OR DIVIDED CAVITIES PENICILLUS DENTALIUM VERMICU- 
LARIA SERPULA SILIQUARIA. 
Having now arrived at those shells which are formed into nearly 
straight, or partly spiral and partly straight, or irregularly contorted 
tubes, some of which are inhabited by vermes, and not by inolluscce, 
I have concluded it to be better to introduce these shells together here, 
than to separate them according to the difference of their inhabitants. 
This seems to be particularly proper, since several of these shells, as will 
be seen, are known only as fossils, and consequently we can form no de- 
termined judgment respecting the animals which formed and inhabited 
them. Besides, as several of these shells are concamerated, their exami- 
nation will very naturally precede that of the shells of the next division. 
LXVII. Penicillus. A tubular shell, narrow, and rather spirally 
turned at its origin, dilating into a club -form at the other end, which 
terminates in a convex disk, beset with small tubular perforations. 
This shell, which is Serpula penis, Linn, has not been seen fossil ; 
nor is the nature of its inhabitant known. 
LXVIII. Dentalium. A tubular, conical, slightly bowed univalve, 
open at both ends. 
Mr. Brander found, among the Hampton fossils, two species ; the 
one of which he considered as I). elephantinum, and the other D. 
entalis. A specimen before me, of the latter fossil, seems in no re- 
spect to differ from the recent shell. Another of my Hampton fossils 
seems to possess the characters of D. dentalis. 
