28 
Northcutt 
plate is asymmetrical, with the lateral edge 
being much thicker, 
Xantusia is characterized by a medial 
ridge plate consisting of a solid sheet of 
small cells with densely staining nuclei. The 
medial plate is bounded laterally by a region 
of large cells whose density is about half 
that of the core. This region may be a spe- 
cialized portion of the medial ridge plate and 
may represent a differential migration of 
the giant cells. Lialis also possesses a similar 
specialized ridge area, but giant cells can still 
be seen within the peripheral plate. Experi- 
mental studies are needed to further analyze 
possible ridge specializations in these taxa. 
The caudal ridge of Xantusia is almost 
identical to those of gekkonids and Spheno- 
don. Distinct cell clusters are arranged in a 
single row or peripheral plate, and nucleus 
sphericus is located immediately ventral. 
Nucleus sphericus is moderately developed 
in Xantusia as it is in the gekkonids. How- 
ever, nucleus sphericus in Lialis is better 
developed than in Xantusia or in gekkonids. 
The DVR of lacertids and scincids are 
very similar. Both possess a clearly defined 
peripheral plate and central core. The 
peripheral plate is asymmetrical as seen in 
the transverse plane. In both families, the 
medial plate (area A) is at least half the 
thickness of the lateral plate (area C), The 
lateral plate is better developed in lacertids 
than in scincids, but in both families the 
cells of the lateral plate show marked clus- 
ters, and giant cells (2-3 times the diameter 
of the other plate cells) are scattered 
throughout the plate. The core cells are 
smaller than the plate cells, and the core is 
about as thick as the plate when seen in the 
transverse plane. The marked asymmetry in 
the ridge plate of both families suggests that 
vision is more extensively represented in the 
ridge than audition, and that lacertids pos- 
sess a larger ridge visual area than do scin- 
cids. Dihamus, Anelytropsis, Typhlosaurus, 
and Feylinia, taxa closely related to scincids, 
all possess extremely reduced ridge areas 
and have been discussed in detail elsewhere 
(Senn and Northcutt, 1973) . 
The anguids {Anguis, Anniella, Gerrhono- 
tus, and Ophisaurus) possess an even more 
extreme asymmetry in the DVR, In all these 
taxa, the rostral medial plate of the ridge 
(area A) consists of a plate of cells in con- 
tact with the ependymal lining of the ventri- 
cle. In these taxa, the plate of area A is 2-8 
cells thick, while the core may be 18 times 
greater in diameter than the plate. At rostral 
levels, the cells of area A form a solid plate, 
while more caudally the cells cluster as in 
gekkonids. The lateral plate is usually 5 
times as thick as the medial plate, its cells 
form the usual clusters, and giant cells are 
particularly obvious in the lateral plate. In 
anguids, the core is at least twice the thick- 
ness of the lateral plate, and giant cells are 
located in the core of these taxa. 
Anniella exhibits an extreme case among 
anguids in that rostrally the entire ridge 
plate, as seen in transverse plane, is reduced 
to a periventricular layer, 3-5 cells thick. 
However, even in Anniella, giant cells are 
scattered among the smaller plate cells, the 
ridge does not appear reduced in volume, and 
an extensive core is present. More caudally, 
the plate has migrated away from the epen- 
dyma, and the cells are scattered with little 
tendency to form clusters. 
The DVR of Xenosaurus closely resembles 
that of anguids. Rostrally, the medial plate 
(area A) is 2-3 cells thick and in contact 
with the ependyma, while the lateral plate 
(area C) is considerably thicker and charac- 
terized by the usual cell clusters. More cau- 
dally, the entire plate migrates centrally and 
forms a layer 4-5 cells thick with little tend- 
ency to form cell clusters. The core is ex- 
tensive throughout and is 4-5 times as thick 
as the peripheral cell plate. The core cells 
are slightly smaller than the plate cells, but 
the difference in their size is not as great as 
in lacertids and scincids. Nucleus sphericus 
is not as well developed in Xenosaurus as in 
the anguids, lacertids, and scincids. 
The cordylids possess large ridges with 
pronounced cell clusters throughout, and the 
cells of the plate and core are approximately 
the same size. The plate can be distinguished 
from the core primarily by a greater density 
of cells. The core is at least twice as thick as 
