powershell general collections

i have pulling .net horizon powershell, i have strike something i don't understand. works fine:

$foo = new-object "system.collections.generic.dictionary``2[system.string,system.string]"
$foo.add("foo", "bar")
$foo

key                                                         value
---                                                         -----
foo                                                         bar

this however does not:

$bar = new-object "system.collections.generic.sorteddictionary``2[system.string,system.string]"
new-object : can't form [system.collections.generic.sorteddictionary`2[system.string,system.string]]: certain t
he public containing form loaded.
at line:1 char:18
+ $bar = new-object <<<< "system.collections.generic.sorteddictionary``2[system.string,system.string]"

they both same assembly, am i missing?

as separate out answers, graceful many wholly an emanate powershell v1.