scala> class A; class B extends A; class C extends A;
defined class A
defined class B
defined class C

scala> val bs = (0 to 5) map { i => new B }
bs: scala.collection.immutable.IndexedSeq[B] = Vector(B@3a5367af, B@39bc3013, B@71f84d01, B@6487b71b, B@a06812d, B@310ab67f)

scala> val c = new C
c: C = C@2e71edc0

scala> c :: bs
<console>:11: error: value :: is not a member of scala.collection.immutable.IndexedSeq[B]
       c :: bs
            ^

scala> c :: bs.toList
res9: List[A] = List(C@2e71edc0, B@3a5367af, B@39bc3013, B@71f84d01, B@6487b71b, B@a06812d, B@310ab67f)

scala>