DENTALIAD #. 225 
GASTEROPODA. 
That this division of the Mollusca is the sequence of the 
Bivalves is universally acknowledged ; this is shown by a body 
of facts that are familiar to all who cultivate this branch of 
science ; but it is also evident that the Gasteropoda have very 
decided marks of an advancement to superior organization ; 
the distinct head, eyes, tentacula, denticular apparatus, the 
cesophagean concentration of the medullary ganglions, the 
character of the foot, the almost invariable state of freedom 
and consequent increased facility of locomotion, together with 
a greater complexity of the generative developments of many 
of the families, are sufficient proofs of a great progress in 
animalization. 
It is not necessary here to enter on anatomical remarks, as 
almost every pomt connected both with the internal and ex- 
ternal animal structure is again and again mentioned in the 
observations concerning the different families ; it would there- 
fore be a useless repetition to extend our notes in this 
direction, and we at once proceed to introduce the respective 
families, only premising that there are still some gaps in the 
descriptions of the animals, but these deficiencies are daily 
disappearing in consequence of many recent discoveries. 
DENTALIAD &. 
We are obliged to provide for this family of one genus, 
a distinct branchial section, which, from the symmetrical 
lateral disposition of the branchiz, we have termed Lateri- 
branchiata; we consider this arrangement subject to our 
determination of the branchiz being confirmed. Large ana- 
tomical details of the Dentalium Tarentinum are annexed. 
From my observations in the ‘Annals of Natural History,’ 
Q 
