diff --git a/src/parse/psy_assembly.rs b/src/parse/psy_assembly.rs
index 4a1eae6..adced2a 100644
--- a/src/parse/psy_assembly.rs
+++ b/src/parse/psy_assembly.rs
@@ -73,15 +73,15 @@ pub fn parse_assembly<'a>(
 
             // Get object data.
             let object_data = match events.next_event()? {
-                Event::InnerOpen {
-                    type_name: "Assembly",
-                    ..
-                } => ObjectData::Assembly(Box::new(parse_assembly(arena, events)?)),
-
                 Event::InnerOpen {
                     type_name: "MeshSurface",
                     ..
-                } => ObjectData::Surface(Box::new(parse_mesh_surface(arena, events)?)),
+                } => ObjectData::Surface(Box::new(parse_mesh_surface(
+                    arena,
+                    &instance_xform_idxs[..],
+                    &assembly.xforms[..],
+                    events,
+                )?)),
 
                 Event::InnerOpen {
                     type_name: "SphereLight",
diff --git a/src/parse/psy_mesh_surface.rs b/src/parse/psy_mesh_surface.rs
index 0bd7cbd..069df75 100644
--- a/src/parse/psy_mesh_surface.rs
+++ b/src/parse/psy_mesh_surface.rs
@@ -9,7 +9,7 @@ use kioku::Arena;
 use data_tree::{DataTreeReader, Event};
 
 use crate::{
-    math::{Normal, Point},
+    math::{Matrix4x4, Normal, Point},
     surface::triangle_mesh::TriangleMesh,
 };
 
@@ -18,15 +18,10 @@ use super::{
     psy::{PsyError, PsyResult},
 };
 
-// pub struct TriangleMesh {
-//    time_samples: usize,
-//    geo: Vec<(Point, Point, Point)>,
-//    indices: Vec<usize>,
-//    accel: BVH,
-// }
-
 pub fn parse_mesh_surface<'a>(
     arena: &'a Arena,
+    instance_xform_idxs: &[std::ops::Range<usize>],
+    xforms: &[Matrix4x4],
     events: &mut DataTreeReader<impl BufRead>,
 ) -> PsyResult<TriangleMesh<'a>> {
     let mut verts = Vec::new(); // Vec of vecs, one for each time sample
diff --git a/src/scene/assembly.rs b/src/scene/assembly.rs
index dff79f5..87c177d 100644
--- a/src/scene/assembly.rs
+++ b/src/scene/assembly.rs
@@ -2,6 +2,7 @@ use std::{collections::HashMap, ops::Range};
 
 use crate::{light::SurfaceLight, math::Matrix4x4, surface::Surface};
 
+/// Stores the objects of a scene and its acceleration structures.
 #[derive(Debug)]
 pub struct Assembly<'a> {
     pub objects: HashMap<String, Object<'a>>, // Name, Object.
@@ -21,7 +22,8 @@ impl<'a> Assembly<'a> {
 pub struct Object<'a> {
     pub data: ObjectData<'a>,
 
-    // One range per instance, indexing into the assembly's xforms array.
+    // One range per instance, indexing into the assembly's xforms
+    // array.  An empty Vec means a single instance with no transforms.
     pub instance_xform_idxs: Vec<Range<usize>>,
 }
 
@@ -30,5 +32,4 @@ pub enum ObjectData<'a> {
     Empty,
     Surface(Box<dyn Surface + 'a>),
     Light(Box<dyn SurfaceLight + 'a>),
-    Assembly(Box<Assembly<'a>>),
 }