100 JouRNAL OF CoMPARATIVE NEUROLOGY. 
system must have a significance, if we can but interpret the 
facts. The system is, primarily, a tactile apparatus for the 
head. Any one who has watched Mustelus exploring with his 
snout every corner of a new aquarium cannot doubt that tactile 
impressions from this region must have a large place in the life 
of the animal. With practically no other check upon his visual 
sensations than can be derived from poking his nose into things, 
widely spread central terminations of cutaneous fibres is no, 
more than should be expected. 
7. The Tuberculum Acusticum. 
The tuberculum acusticum is the ¢rigemznal lobe of V1IAULT 
(76), Ronon ('77), and SaNnpErs (’86); Kincspury (’97) has 
extended the term cerebellar crest to the entire structure; while 
JOHNSTON (’98b) appears to include under his taberculum acustr- 
cum all those structures in the oblongata which are homologous 
with the dorsal cornu of the spinal cord. The term ¢tzgeminal 
Jobe has been so variously used that it should be dropped from 
our nomenclature. The earlier writers on the brain of the 
selachian so designated the tuberculum acusticum, supposing, 
erroneously so, that the great trigeminal complex, which 
emerges just beneath its anterior end, must have the origin of 
its nerve-roots here: 
The tuberculum acusticum is the primary terminal station 
_for the acustico-lateral system. Fibres from the lateral line 
sense-organs and the internal ear are components of the VII, 
VIII, and X nerves. * 
The structure of the tuberculum acusticum embraces a me- 
dio-central core in which bundles of nerve-fibres predominate ; 
and an outer investing cap, the cerebellar crest (Fig. 2, cb. cr.). 
The latter, in its outer levels, is morphologically continuous 
with the molecular layer of the cerebellum, so that the term 
molecular layer is very properly applied here. The neurones 
of the tuberculum acusticum lie chiefly in the cerebellar crest, 
with just a few, also, in the central core. Three varieties are 
to be distinguished: PURKINJE neurones, granular neurones, 
and molecular neurones. I have so designated them because 
