163 
B. proripiens: 3% cirrus, longer ramus has a group of 2—4 short strong teeth near anterior 
face, and upper extremity of all the segments. 
4" cirrus, shorter ramus has the 5 to 6 lower segments armed with a group of from 
2 to 4 small teeth. 
Finally, a remark regarding the relations between the fossil and recent forms, and 
another regarding the species living at different depths, may find a place here. 
Darwin" states that Laldanus is the oldest genus of the Balanidae yet known; according 
to him, the extinct forms of the genus belong to the section that has the parietes not 
permeated by pores. He therefore seems to admit, that the oldest species were those that 
have no pores. Consequently, the fact that the oldest-known species — B. unguiformis from 
the Eocene — had the parietes almost as often as not. permeated by pores is considered 
by Darwin as presenting to the systematist a most unfortunate peculiarity. However, when we 
consider that of the ten other fossil British species, which Darwin described, 6 have the walls 
permeated, and 4 have no pores in the walls, we may certainly ask whether this character 
has really the importance Darwin gives it. It may be true, that of these four species, three 
(B. bctsutcatus, B. dolosus, and &. inclusus) are extinct forms, whereas all the other fossil 
species he describes are also found living. But this, I think, does not prove that these three 
species are really older or more primitive forms. The forms still existing are found along with 
the extinct ones throughout the same tertiary stages: special circumstances, which we need 
not consider here, may have caused the equally old or even older species to remain, and others 
as old or possibly not so old to become extinct. The fact that the oldest-known species of 
Lalanus (BL. ungutformis), sometimes has the parietes permeated by pores, goes far to prove 
that the permeated condition of the walls is as old as the genus itself. 
In this connection it is interesting to observe that the species which inhabit deeper 
water have the walls as a rule thin, seldom thick or heavy, and sometimes longitudinally ribbed 
on the internal surface, sometimes not, but almost always solid, and without pores. At very 
considerable depths, no species of this genus seem to occur, but those found in rather deep water 
must, I think, be regarded as younger forms which have developed from shallow water species. 
It even seems possible, in some instances, to point out the shallow water species’ from which a 
form of the deeper sea may have developed: in such a case it may be a species with permeated 
parietes from which another with solid walls may have descended. The species living at greater 
depth do not want such strong shells as those that are continually exposed to the rolling 
waves near the coast; so the permeated condition of the walls, which is an ideal structure as 
regards strength and economy in the use of shelly matter, and which has its highest develop- 
ment in numerous true coastal forms, may have been lost by a series of forms gradually 
wandering into deeper water. I need hardly add that, according to my opinion, there is 
no reason to consider the species from deeper water as representing more primitive forms. 
On the other hand, species with solid walls are also observed in shallow water and, of 
1 Darwin, CH., A Monograph on the fossil Balanidae and Verrucidae of Great Britain. London. 1854. p. 5. 
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