244 PROFESSOR NICHOLSON ON THE MODE OF GROWTH AND INCREASE 
LonsDALE, for example, regards the well known Chetetes petropolitanus, Pander, 
as increasing fissiparously, and consequently places it in the amended genus 
Chetetes. MiLNE-Epwarps and JuLes Harmg, on the other hand, regard the 
- game species as increasing by gemmation, and consequently refer it to the genus 
Monticulipora. If this discrepancy of opinion is possible about such a large, 
massive, and common species as C. petropolitanus, we must conclude that this 
distinction is quite valueless when we have to deal with many of the minute 
corals which belong to one or other of these groups. Upon the whole, there- 
fore, even if we were to admit as a matter of theory that the mode of growth 
is of itself sufficient to constitute a generic distinction, we should be forced to 
conclude that the difficulties in the way of its application to many fossil corals 
are so great, that it becomes practically useless. It may be added in this con- 
nection, that in the corals which belong to Chetetes or Monticulipora, in which 
gemmation is clearly the chief or only mode of increase, the method in which 
gemmation is carried out, nevertheless, differs very widely in different species. 
- Thus, the thin encrusting forms, like C. Ortoni, Nich., and C. quadrangularis, 
Nich., undoubtedly increase by basal or marginal gemmation alone. The ramose 
or dendroid species, like C. pulchellus, Edw. and H., C. Fletcheri, Edw. and 
H., and other similar forms, increase by lateral or interstitial gemmation. The 
more massive forms, like C. petropolitanus, Pander, appear to increase by a 
combination of lateral with basal gemmation. Finally, the strictly frondescent 
species, such as C. mammulatus, Edw. and H., and C. clathratulus, James, which 
consist of two layers of corallites directed in opposite directions from a median 
plane, appear to increase wholly by basal gemmation. 
Again, considerable difficulty has been met with in the separation of the 
genera Lithostrotion and Diphyphyllum, in consequence of the fact that the 
mode of increase has been taken as one of the most important elements of the 
generic diagnosis. In the forms now usually referred to Lithostrotion, the mode 
of increase is clearly by lateral gemmation, the resulting corallum being some- 
times compact and astreiform, sometimes fasciculate. The genus Diphyphyllum 
was founded by LonspaALE for forms which closely resemble the fasciculate 
species of Lithostrotion, but which this eminent paleontologist believed to in- 
crease fissiparously. It is now certain, however, that the mode of increase in 
Lithostrotion and Diphyphyllum is identical, both increasing by lateral gemma- 
tion. The real distinction between the two genera is to be found in the fact 
that a columella is present in Lithostrotion, whilst this structure, in spite of the 
doubts of MiLtnE-Epwarps and Harms, is certainly absent in Diphyphyllum. 
It seems wise, however, to conclude that a poimt upon which the most eminent 
authorities may differ should not be employed prominently in determining the 
generic position of fossil corals, the difficulties in the way of its practical appli- 
cation being so great as to preclude its use for this purpose. 
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