40 
Northcutt 
ridge (Foster, 1974; Fritz, 1974a, 1974&; 
Distel and Ebbesson, 1975). 
Nucleus medialis posterior is located cau- 
dal to nucleus rotundus and forms the most 
caudal division of the dorsal thalamus (Fig. 
11). This nucleus receives an ascending- 
spinal projection in Caiman and Tupinambis 
(Ebbesson, personal communication) ; in 
Caiman, at least, it projects to a medial com- 
ponent of the dorsal ventricular ridge (un- 
published observations). This circuit most 
likely represents an ascending somatic sen- 
sory path-way found in most reptiles ; a similar 
path-way has been reported in birds (Delius 
and Bennetto, 1972). 
T-wo distinct dorsal thalamic patterns can 
be recognized in lizards. Agamids, chamae- 
leonids, iguanids, teiids, and varanids possess 
an expanded nucleus dorsolateralis and nu- 
cleus rotundus. The enlargement of these 
nuclei is correlated -with a displacement of 
the medial dorsal thalamus. In these taxa, 
nucleus rotundus frequently reaches the 
ependymal layer; nucleus dorsomedialis is 
displaced dorsally, and nucleus medialis dis- 
placed ventrally. However, the density of cells 
forming nucleus rotundus does not appear to 
increase in the above taxa. Additionally, 
both the dorsal and ventral thalamic nuclei 
that receive direct retinal projections, as well 
as the auditory relay nucleus (nucleus medi- 
alis), are enlarged in these taxa. The remain- 
ing families of lizards possess a thalamic 
pattern that is very similar to that illus- 
trated for gekkonids (Figs. 10, 11), or 
Sphenodon. 
Thalamic analysis beyond this level is ex- 
tremely difficult due to the extended length 
of most of the thalamic nuclei that compli- 
cates direct comparisons, and the probability 
of error in such analysis would be great 
without quantitative measurements. How- 
ever, a few qualitative observations can be 
made with some confidence. Anelytropsis, 
Anniella, Dibamus, Feylinia, and Typhlo- 
saurus show reductions or loss (i.e., an in- 
ability on the part of an observer to recog- 
nize discrete neural population) of the lateral 
optic nuclei. Surprisingly the microteiids, 
with the exception of Bachia and Ophiog- 
nomon, do not exhibit reductions in the 
thalamic visual nuclei^ and are indistinguish- 
able from the macroteiids. Gekkonids and 
Xantusia appear to have an enlarged nucleus 
dorsolateralis, but not the other dorsal 
thalamic nuclei. In these taxa, dorsolateralis 
extends far ventral and medial to nucleus 
rotundus (Fig. 10). In Xantusia, nucleus 
dorsolateralis actually fuses across the mid- 
line, as does nucleus medialis. 
At present, there is every reason to believe 
that the diencephalic organization of lizards 
exhibits as much variation as the telencepha- 
lon or tectum. However, detailed analysis of 
this brain region requires further study and 
quantitative data. 
Pretectum 
The pretectum of lizards, like that of other 
vertebrates, occupies the dorsal and lateral 
walls of the caudal diencephalon. It is a 
transitional area bordered rostrally by the 
posterior commissure of the thalamus and 
caudally by the intertectal commissure. In 
lizards, as well as other reptiles, it consists 
of three nuclear zones extending from ven- 
tricular to pial surfaces. The most medial 
cell group is termed nucleus lentiformis 
thalami (Figs. 11, 13, 14). This nucleus is 
divided into a medial part, the pars plicata, 
and a lateral part, pars extensa, by the 
lateral continuation of the posterior com- 
missure. At present nothing is known regard- 
ing the connections of pars plicata. In most 
lizards the pars extensa forms a lens-shaped 
nucleus that extends across the pretectum 
as seen in the transverse plane (Fig. 13). 
Its lateral edge is marked by a compact group 
of large bipolar cells which forms the cell 
plate of nucleus geniculatus pretectalis. 
In representative taxa of many families 
(Gekkonidae, Dibamidae, Lacertidae, Xan- 
tusiidae, Anguidae, Xenosauridae, Cordy- 
lidae, Gerrhosauridae, Lanthanotidae, and 
Helodermatidae) the pars extensa is a rather 
uniform scattered nuclear field in which it 
is possible to recognize dorsolateral and 
ventromedial subdivisions (Fig. 14A). In 
