diff --git a/shardtree/src/lib.rs b/shardtree/src/lib.rs
index e5cdc8d..14b56a7 100644
--- a/shardtree/src/lib.rs
+++ b/shardtree/src/lib.rs
@@ -1420,37 +1420,54 @@ pub trait ShardStore<H> {
     fn truncate(&mut self, from: Address) -> Result<(), Self::Error>;
 }
 
-impl<H> ShardStore<H> for Vec<LocatedPrunableTree<H>> {
+#[derive(Debug)]
+pub struct MemoryShardStore<H> {
+    shards: Vec<LocatedPrunableTree<H>>,
+}
+
+impl<H> MemoryShardStore<H> {
+    pub fn empty() -> Self {
+        Self { shards: vec![] }
+    }
+}
+
+impl<H> ShardStore<H> for MemoryShardStore<H> {
     type Error = Infallible;
 
     fn get_shard(&self, shard_root: Address) -> Option<&LocatedPrunableTree<H>> {
-        self.get(usize::try_from(shard_root.index()).expect("SHARD_HEIGHT > 64 is unsupported"))
+        let shard_idx =
+            usize::try_from(shard_root.index()).expect("SHARD_HEIGHT > 64 is unsupported");
+        self.shards.get(shard_idx)
     }
 
     fn last_shard(&self) -> Option<&LocatedPrunableTree<H>> {
-        self.last()
+        self.shards.last()
     }
 
     fn put_shard(&mut self, subtree: LocatedPrunableTree<H>) -> Result<(), Self::Error> {
         let subtree_addr = subtree.root_addr;
-        for subtree_idx in self.last().map_or(0, |s| s.root_addr.index() + 1)..=subtree_addr.index()
+        for subtree_idx in
+            self.shards.last().map_or(0, |s| s.root_addr.index() + 1)..=subtree_addr.index()
         {
-            self.push(LocatedTree {
+            self.shards.push(LocatedTree {
                 root_addr: Address::from_parts(subtree_addr.level(), subtree_idx),
                 root: Tree(Node::Nil),
             })
         }
-        self[usize::try_from(subtree_addr.index()).expect("SHARD_HEIGHT > 64 is unsupported")] =
-            subtree;
+
+        let shard_idx =
+            usize::try_from(subtree_addr.index()).expect("SHARD_HEIGHT > 64 is unsupported");
+        self.shards[shard_idx] = subtree;
         Ok(())
     }
 
     fn get_shard_roots(&self) -> Vec<Address> {
-        self.iter().map(|s| s.root_addr).collect()
+        self.shards.iter().map(|s| s.root_addr).collect()
     }
 
     fn truncate(&mut self, from: Address) -> Result<(), Self::Error> {
-        self.truncate(usize::try_from(from.index()).expect("SHARD_HEIGHT > 64 is unsupported"));
+        let shard_idx = usize::try_from(from.index()).expect("SHARD_HEIGHT > 64 is unsupported");
+        self.shards.truncate(shard_idx);
         Ok(())
     }
 }
@@ -2224,8 +2241,8 @@ pub mod testing {
 #[cfg(test)]
 mod tests {
     use crate::{
-        IncompleteAt, LocatedPrunableTree, LocatedTree, Node, PrunableTree, QueryError,
-        RetentionFlags, ShardStore, ShardTree, Tree,
+        IncompleteAt, LocatedPrunableTree, LocatedTree, MemoryShardStore, Node, PrunableTree,
+        QueryError, RetentionFlags, ShardStore, ShardTree, Tree,
     };
     use assert_matches::assert_matches;
     use core::convert::Infallible;
@@ -2390,8 +2407,6 @@ mod tests {
         );
     }
 
-    type VecShardStore<H> = Vec<LocatedPrunableTree<H>>;
-
     #[test]
     fn tree_marked_positions() {
         let t: PrunableTree<String> = parent(
@@ -2554,8 +2569,8 @@ mod tests {
 
     #[test]
     fn shardtree_insertion() {
-        let mut tree: ShardTree<String, usize, VecShardStore<String>, 4, 3> =
-            ShardTree::new(vec![], 100, 0);
+        let mut tree: ShardTree<String, usize, MemoryShardStore<String>, 4, 3> =
+            ShardTree::new(MemoryShardStore::empty(), 100, 0);
         assert_matches!(
             tree.batch_insert(
                 Position::from(1),
@@ -2701,35 +2716,55 @@ mod tests {
     #[test]
     fn append() {
         check_append(|m| {
-            ShardTree::<String, usize, VecShardStore<String>, 4, 3>::new(vec![], m, 0)
+            ShardTree::<String, usize, MemoryShardStore<String>, 4, 3>::new(
+                MemoryShardStore::empty(),
+                m,
+                0,
+            )
         });
     }
 
     #[test]
     fn root_hashes() {
         check_root_hashes(|m| {
-            ShardTree::<String, usize, VecShardStore<String>, 4, 3>::new(vec![], m, 0)
+            ShardTree::<String, usize, MemoryShardStore<String>, 4, 3>::new(
+                MemoryShardStore::empty(),
+                m,
+                0,
+            )
         });
     }
 
     #[test]
     fn witnesses() {
         check_witnesses(|m| {
-            ShardTree::<String, usize, VecShardStore<String>, 4, 3>::new(vec![], m, 0)
+            ShardTree::<String, usize, MemoryShardStore<String>, 4, 3>::new(
+                MemoryShardStore::empty(),
+                m,
+                0,
+            )
         });
     }
 
     #[test]
     fn checkpoint_rewind() {
         check_checkpoint_rewind(|m| {
-            ShardTree::<String, usize, VecShardStore<String>, 4, 3>::new(vec![], m, 0)
+            ShardTree::<String, usize, MemoryShardStore<String>, 4, 3>::new(
+                MemoryShardStore::empty(),
+                m,
+                0,
+            )
         });
     }
 
     #[test]
     fn rewind_remove_mark() {
         check_rewind_remove_mark(|m| {
-            ShardTree::<String, usize, VecShardStore<String>, 4, 3>::new(vec![], m, 0)
+            ShardTree::<String, usize, MemoryShardStore<String>, 4, 3>::new(
+                MemoryShardStore::empty(),
+                m,
+                0,
+            )
         });
     }
 
@@ -2741,11 +2776,11 @@ mod tests {
         H,
         usize,
         CompleteTree<H, usize, 4>,
-        ShardTree<H, usize, VecShardStore<H>, 4, 3>,
+        ShardTree<H, usize, MemoryShardStore<H>, 4, 3>,
     > {
         CombinedTree::new(
             CompleteTree::new(max_checkpoints, 0),
-            ShardTree::new(vec![], max_checkpoints, 0),
+            ShardTree::new(MemoryShardStore::empty(), max_checkpoints, 0),
         )
     }