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II. Tribe,x—Holostomata. 
Under the name of Hotosromata are included those Clenobranchiate Gastro- 
poda, whish have an external, calcareous, cup-shaped or spiral shell, the aperture of 
which is not anteriorly produced into a canal, and very rarely emarginated, or indis- 
tinctly notched ; having, as a rule, an uninterrupted margin in front ; the spiral shells 
possess generally a horny or calcareous operculum. 
Although the animals of the CTENOBRANCHIATA HOLOSTOMATA often closely 
resemble those of the Cr. stPpHoNostomaTA, they are very rarely provided with a 
prolonged respiratory sipho. A large number of them, however, have in its place 
a more or less distinct siphonal fold in the anterior margin of the mantle. Upon 
the development of this fold depends, to some extent, the formation of the anterior 
portion of the aperture in the shell. Thus, while most of the HoLostomata, as 
stated, have the margin of the aperture in front entire, we find among the 
uecanoPsin® (of the family Mzzanip#) and among the Pravaxrpz# specimens, 
which have the aperture anteriorly more distinctly emarginated, than many 
Crriraip2#, though the siphonal fold itself is hardly so well marked, as in the last 
named family. The Huozzurpz, Sorarimp., and others have, under similar cireum- 
stances, the aperture only slightly produced without a notch or an emargination. 
The development of along sipho in Ampullaria, not being connected in any way 
with the usual emargination of the aperture, appear somewhat anomalous, and is 
probably due to the peculiar mode of living. These and similar cases, concerning 
other parts of the organisation of the animals, and the formation of the shells, 
exhibit clearly the shortcomings of the system here adopted. It is hardly necessary 
on my part to repeat, what I have previously stated, namely, that I regard this 
entire classification of the PRosOBRANCHIA merely as a temporary one. It is far 
from natural; but I have retained it (though with some modifications in the arrange- 
ment and characteristics of the different groups), because it has for the palzon- 
tologist certain advantages, which are at the present not easily obtained from other 
systems, based solely upon the dentition. The exceptions from the general charac- 
teristics of the orders, tribes, and other groupings, are not so numerous, as would 
require to be recorded in the systems of Gray, H. and A. Adams, and others. 
Tracing out the Geological history of the CreNoBRANCHIATA, the HoLostomata 
must be regarded as much older than the SrpHonostomata. For while the latter 
appear only very sparingly at the beginning of the mesozoic period, the former were — 
already tolerably numerous in the palzeozoic, and have probably their maximum in 
the first half of the former. 
Looking at the general results of the palzeontological researches, according to 
which the older forms in the different classes of animals are generally lower in 
organisation, the early appearance of the Honosromata agrees tolerably well with 
their lower place in the system. This classification is partly supported by the 
somewhat defective development of their organs of locomotion, respiration, the 
uniformity of dentition, of generation, &c., in many families or groups, as compared 
with those of the SIPHONOSTOMATA. 
3 E 
