214 Mr. Toulmin Smith on the Classification 
research. The fact of the mammillated external appearance of 
this species is easily seen and as easily recorded : so is also the 
fact of the striated internal appearance. But it would be difficult, 
by any description, to make it clearly understood how such ap- 
parently contradictory appearances could result from the folding 
of a simple membrane*, until it had been ascertained, by careful 
and multiplied dissections, that this and all other characters of 
the fold of the membrane forming the wall of the pouch are 
grounded upon one single and simple unity of plan of which all 
present clear and intelligible modifications. 
On the outside of this species as usually seen in a fossil state, 
and as it must usually have appeared in a recent state, no mark 
of the plait is seen. Large rounded elevations scattered, at first 
sight irregularly, over the whole surface, are all that meet the 
eye. If however the reader will carefully consider figs. 13 and 
14 of PI. XIII. he will see, in fig. 13, the simple plaits, regular and 
uninterrupted, within and without, as in V. striatus’, while in 
fig. 14 he will see that the perfect plait is still present, and that 
the inner surface [the lower part of the figure] is still simple and 
uninterrupted, but that the outer plaits are interrupted along 
their whole length by rounded elevations, — not solid, but hollow, 
— ^elevations of the membrane into those shapes instead of the 
plait being plain and simple. These figures are somewhat ex- 
aggerated in size in order that the principle may be more clearly 
understood. 
It will be found that in each of the forms which follow, and 
* As tlie extraordinary complications in the fold of the membrane of 
many species were gradually developed by multiplied dissections, 1 long de- 
spaired of being able, by any descriptions, to make them understood. When 
by degrees the unity of plan which I have endeavoured to indicate above, 
as the groundwork of all that complexity, opened upon me, I felt an im- 
portant key to have been obtained, which experiment proved to be appli- 
cable to every case. I indulge some hope that the development of this 
unity may be of a utility beyond the mere understanding of the forms now 
under discussion — however interesting those may be to myself and others. 
In the descriptions of numerous tissues in human and other branches of 
anatomy, I have often myself felt the v^ant of some clear and simple basis 
of unity in the descriptions attempted of those tissues ; which want has 
caused them to be often unintelligible. Any pains which the investigation 
of the present subject may have cost will be more than rewarded if the sug- 
gestion of an unfailing unity in the arrangement of all complex tissues shall 
be felt to be (as I cannot doubt that it is) generally applicable. It seems to 
me, that as, by application of this principle, forms varying so entirely as it 
will be found that many of the Ventriculidae do in mere general external cha- 
racters, and therefore heretofore classed in entirely different natural groups, 
are now demonstrated to belong to one group, so the application of the like 
principle may, in many points of anatomy, lead to the discovery of intimate 
relations, not now suspected, between tissues which appear very different 
in structtire. 
