Why are several login shells missing in Ubuntu?

I am new to Ubuntu. I just recently installed Ubuntu 10.04.1
alongside Mac OS X to have a dual-boot system. I know I
have a LOT to learn about Linux! That being said, when I
tried to do: chsh -s /bin/tcsh I got an error message. The
file /etc/shells contains:

# /etc/shells: valid login shells
/bin/csh
/bin/sh
/usr/bin/es
/usr/bin/ksh
/bin/ksh
/usr/bin/rc
/usr/bin/tcsh
/bin/tcsh
/usr/bin/esh
/bin/dash
/bin/bash
/bin/rbash
/usr/bin/screen

But MOST of these shells are missing:

ls /bin/csh
ls: cannot access /bin/csh: No such file or directory

ls /bin/sh
/bin/sh

ls /usr/bin/es
ls: cannot access /usr/bin/es: No such file or directory

ls /usr/bin/ksh
ls: cannot access /usr/bin/ksh: No such file or directory

ls /bin/ksh
ls: cannot access /bin/ksh: No such file or directory

ls /usr/bin/rc
ls: cannot access /usr/bin/rc: No such file or directory

ls /usr/bin/tcsh
ls: cannot access /usr/bin/tcsh: No such file or directory

ls /bin/tcsh
ls: cannot access /bin/tcsh: No such file or directory

ls /usr/bin/esh
ls: cannot access /usr/bin/esh: No such file or directory

ls /bin/dash
/bin/dash

ls /bin/bash
/bin/bash

ls /bin/rbash
/bin/rbash

ls /usr/bin/screen
/usr/bin/screen

Why are these missing from Ubuntu? Can I get them? How?

Thanks

Ron