22 
centralization and individuality of the organ would lead us to expect, 
many variations and modifications, which tend at first sight to con¬ 
ceal its real nature, as e. g. the removal of the olfactory ganglia to a 
great distance from the other elements of the brain, with which they 
only maintain their connexion by means of filiform crura, as in the 
Whiting and many fish; the amplification of the segments of the 
encephalon by the addition of supplementary ganglia, as the hypo- 
aria, hypophysis, &c. as they occur in many fish, and some of which 
are retained in the higher orders, or the cerebrum in the cartilagi¬ 
nous fishes, and in all animals upwards to man, and which compara¬ 
tive anatomy teaches us is only to be considered as a special appen¬ 
dage to or development of the prosencephalic ganglia; or the ex¬ 
treme development of one pair of ganglia so as to obscure the others, 
as the cerebellum in the Sharks, Sawfish, &c. (Owen’s Lectures, ii. 
175) ; or the very diminutive size of a segment, as the cerebellum in 
many reptiles ; or the coalescence of the pair, and consequent obli¬ 
teration of the mesial division, just as is equally the case between the 
two halves of the spinal cord, as in the cerebellum. 
3. Embryonic anatomy, too, comes in to strengthen the conclusion 
of comparative anatomy, that a series of four ganglia is the essential 
element of the brain, and that all the other parts of which it consists 
in adult life of the higher Vertebrata, including of course the cere¬ 
brum, are superadded. 
The argument of the preceding sections, exclusive of Section I., and 
the conclusion to which it is intended to lead, may thus be stated :— 
Considering that the head of the Insecta, Myriapoda, &c. is com¬ 
posed of a series of segments serially homologous with those of the 
body, as its brain is of ganglia serially homologous with those ot the 
cord; that a vertebra is the general homologue of a segment as the 
spinal cord is of the ganglionic cord ; and that the brain ot the \ er- 
tebrata consists of a series of four segments; there appears a strong 
probability that its head in like manner shall consist of a series of 
four vertebree. 
3. Monograph of the species of Myochama, including 
THE DESCRIPTIONS OF TWO NEW SPECIES FROM THE COL¬ 
LECTION of H. Cuming, Esq. By Arthur Adams, R.N., 
F.L.S. ETC. 
(Mollusca, PI. VIII.) 
Myochama, Stutchbury. 
Testa incequivalvis, adhcerens ; valva affix a dentibus duobus mar- 
ginalibus, divaricatis, ad umbonem disjunctis, foveold trigond 
intermedid alteram testacece appendicis extremitatem, carfima¬ 
gine corned connexam, excipiente ; valva libera dentibus duobus 
incequalibus, parvis, divaricatis, altera appendicis ext remit ate 
foveolce interrnedice insertd ; umbones valvce liberce interne, a/te- 
rins ext erne, recurvi; impressiones musculares duce orbiculares , 
