OF SOUTHERN INDIA. 397 
H. and A. Adams state that, observed under favorable cireumstances, the Mono- 
PLEUROBRANCHIATA are by no means sluggish and unattractive, but that they con- 
tribute, by their changing form and lively colours, to lend animation to the weedy 
shores and the coral reefs among which they take up their abode. 
The following families have been distinguished in this tribe:—- 
LVIII.—ACTHONIDA. 
LIX.—BULLID. 
LX.—OXYNOEID Z. 
LXI.—PHILINEID. 
LXIL.—APLYSTID A. 
LXUL—PLEUROBRANCHIDA. 
LXIV.—RUNCINID 4. 
Ti. and A. Adams, Gray and other conchologists distinguish a somewhat larger or 
smaller number of families, though these do not as yet appear to be well founded. It 
is well known that the animals of the PLEUROBRANCHIATA are very variable in form, 
though they generally agree in the great development of the frontal disk. The 
teeth show slight differences in almost every few species that have been examined, 
and it seems consequently very hazardous to separate closely allied shells, because 
one species happens to have no central teeth, or one or two laterals more, or again 
because the frontal veil is a little more produced, or a little less rounded. There is 
no reason why these characters ought not to be used quite as well as many others, 
but a far more detailed examination of the various living forms is required before 
we are able to come to a definite conclusion as to the arrangements which ought to 
be adopted. I would only remark that in this particular case the characters of 
distinctions are mostly those which exhibit the greatest variations in this group. 
The shells, so far as known, give little support to many sub-divisions. 
The fossil forms belonging to this tribe are rather numerous; the cup-shaped 
forms begin already in the lower paleeozoic epoch, while the spiral forms are not 
with sufficient accuracy known below the carboniferous strata.* They become, how- 
ever, pretty numerous already in the Trias. I shall give brief notes on some of the 
fossil forms when speaking of the separate families; but I may already here remark 
that since several of the fossil genera (as, for instance, Act@onina and others, being 
certainly closely allied to Acte@on), do not always agree with the characteristics of 
the families, as deduced from the recent species alone, some changes in the limits 
of these families must be made. 
It appears hardly necessary to enter into any detailed discussion on the 
three last mentioned families, namely, the 4rzysirp#, PLevroprancHip#, and 
RuNciyIp#, as there are scarcely any eretaceous shells known which can be referred 
to them. The different genera belonging to these families will be found treated in 
H.and A. Adams’ Genera, II, pp. 32-43, and in Gray’s Guide of 1857, p. 198, 
ete. Of the Apzyszp# and the Ruycryz# in particular there are scarcely any 
fossil species known, but of the Pzrevrosrancuip#, especially of the sub-family 
* Actaonina (Chemnitzia) carbonaria, Kon., Anim. foss. terr. carb. de Belgique, pl. 41, fig. 15. 
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