diff --git a/starfish/core/segmentation_mask/segmentation_mask.py b/starfish/core/segmentation_mask/segmentation_mask.py
index 0fb698e36..dbe5a34f8 100644
--- a/starfish/core/segmentation_mask/segmentation_mask.py
+++ b/starfish/core/segmentation_mask/segmentation_mask.py
@@ -1,6 +1,18 @@
 import io
 import tarfile
-from typing import cast, Dict, Hashable, List, Optional, Sequence, Tuple, Union
+from typing import (
+    cast,
+    Dict,
+    Hashable,
+    Iterable,
+    Iterator,
+    MutableMapping,
+    MutableSequence,
+    Optional,
+    Sequence,
+    Tuple,
+    Union,
+)
 
 import numpy as np
 import xarray as xr
@@ -38,11 +50,11 @@ def _validate_segmentation_mask(arr: xr.DataArray):
 
 
 class SegmentationMaskCollection:
-    """Collection of binary segmentation masks with a list-like access pattern.
+    """Collection of binary segmentation masks with a dict-like access pattern.
 
     Parameters
     ----------
-    masks : List[xr.DataArray]
+    masks : Iterable[xr.DataArray]
         Segmentation masks.
 
     Attributes
@@ -50,42 +62,35 @@ class SegmentationMaskCollection:
     max_shape : Dict[Axes, Optional[int]]
         Maximum index of contained masks.
     """
-    def __init__(self, masks: List[xr.DataArray]):
-        self._masks: List[xr.DataArray] = []
+    def __init__(self, masks: Iterable[xr.DataArray]):
+        self._masks: MutableMapping[int, xr.DataArray] = {}
         self.max_shape: Dict[Axes, int] = {
             Axes.X: 0,
             Axes.Y: 0,
             Axes.ZPLANE: 0
         }
 
-        for mask in masks:
-            self.append(mask)
+        for ix, mask in enumerate(masks):
+            _validate_segmentation_mask(mask)
+            self._masks[ix] = mask
 
-    def __getitem__(self, index):
+            for axis in Axes:
+                if axis.value in mask.coords:
+                    max_val = mask.coords[axis.value].values[-1]
+                    if max_val >= self.max_shape[axis]:
+                        self.max_shape[axis] = max_val + 1
+
+    def __getitem__(self, index: int) -> xr.DataArray:
         return self._masks[index]
 
-    def __iter__(self):
-        return iter(self._masks)
+    def __iter__(self) -> Iterator[Tuple[int, xr.DataArray]]:
+        return iter(self._masks.items())
 
-    def __len__(self):
+    def __len__(self) -> int:
         return len(self._masks)
 
-    def append(self, mask: xr.DataArray):
-        """Add an existing segmentation mask.
-
-        Parameters
-        ----------
-        arr : xr.DataArray
-            Segmentation mask.
-        """
-        _validate_segmentation_mask(mask)
-        self._masks.append(mask)
-
-        for axis in Axes:
-            if axis.value in mask.coords:
-                max_val = mask.coords[axis.value].values[-1]
-                if max_val >= self.max_shape[axis]:
-                    self.max_shape[axis] = max_val + 1
+    def masks(self) -> Iterator[xr.DataArray]:
+        return iter(self._masks.values())
 
     @classmethod
     def from_label_image(
@@ -111,8 +116,7 @@ def from_label_image(
 
         dims, _ = _get_axes_names(label_image.ndim)
 
-        masks: List[xr.DataArray] = []
-
+        masks: MutableSequence[xr.DataArray] = []
         coords: Dict[Hashable, Union[list, Tuple[str, Sequence]]]
 
         # for each region (and its properties):
@@ -172,7 +176,7 @@ def to_label_image(
 
         label_image = np.zeros(shape, dtype=np.uint16)
 
-        for i, mask in enumerate(self):
+        for i, mask in iter(self):
             mask = mask.transpose(*[o.value for o in ordering if o in mask.coords])
             coords = mask.values.nonzero()
             j = 0
@@ -218,7 +222,7 @@ def save(self, path: str):
             Path of the tar file to write to.
         """
         with tarfile.open(path, 'w:gz') as t:
-            for i, mask in enumerate(self._masks):
+            for i, mask in iter(self):
                 data = cast(bytes, mask.to_netcdf())
                 with io.BytesIO(data) as buff:
                     info = tarfile.TarInfo(name=str(i) + '.nc')
diff --git a/starfish/core/segmentation_mask/test/test_segmentation_mask.py b/starfish/core/segmentation_mask/test/test_segmentation_mask.py
index 0b18ce294..26137adf1 100644
--- a/starfish/core/segmentation_mask/test/test_segmentation_mask.py
+++ b/starfish/core/segmentation_mask/test/test_segmentation_mask.py
@@ -70,8 +70,8 @@ def test_from_label_image():
     physical_ticks = {Coordinates.Y: [1.2, 2.4, 3.6, 4.8, 6.0],
                       Coordinates.X: [7.2, 8.4, 9.6, 10.8, 12]}
 
-    masks = SegmentationMaskCollection.from_label_image(label_image,
-                                                        physical_ticks)
+    masks = list(SegmentationMaskCollection.from_label_image(
+        label_image, physical_ticks).masks())
 
     assert len(masks) == 2
 
@@ -134,7 +134,7 @@ def test_save_load():
     try:
         masks.save(path)
         masks2 = SegmentationMaskCollection.from_disk(path)
-        for m, m2 in zip(masks, masks2):
+        for m, m2 in zip(masks.masks(), masks2.masks()):
             assert np.array_equal(m, m2)
     finally:
         os.remove(path)
diff --git a/starfish/core/spots/AssignTargets/label.py b/starfish/core/spots/AssignTargets/label.py
index a1ddbadfd..49c897ca1 100644
--- a/starfish/core/spots/AssignTargets/label.py
+++ b/starfish/core/spots/AssignTargets/label.py
@@ -32,7 +32,7 @@ def _assign(
 
         decoded_intensities[Features.CELL_ID] = cell_ids
 
-        for mask in masks:
+        for _, mask in masks:
             y_min, y_max = float(mask.y.min()), float(mask.y.max())
             x_min, x_max = float(mask.x.min()), float(mask.x.max())