377 
M. K. A. Zittel on Fossil Calcispongice. 
sand of Le Mans and Cambridge are remarkable for the 
abundance of beautifully preserved Calcispongise ; and, finally, 
the Cretaceous tuff of Maestricht is the locality which har¬ 
bours the last representatives of the Pharetrones in consider¬ 
able numbers. 
As will be seen from the following Table (p. 378), most of 
the Cretaceous genera agree with those already occurring 
in the Jura. 
It is remarkable that, with the exception of isolated spicules 
in the Red Crag, which are referred by Johnson to Grantia 
compressa , no Calcispongia? have been detected in the Tertiary 
formation ; consequently the Pharetrones, which are evidently 
the most resistant forms, seem to have become extinct at the 
close of the Mesolithic epoch. 
Contrary to what occurs in the siliceous sponges, we find 
in the Pharetrones a tolerably continuous development. 
Many genera overstep the bounds of one or two formations, 
and are remarkable for their unusual longevity. The change 
of form within the genera is also confined within moderate 
limits, so that, under certain circumstances, the species from 
the uppermost Cretaceous present a delusive resemblance to 
those from the Jura and Trias. 
The former mode of life of the Calcispongke probably fur¬ 
nishes the simplest explanation of their closed sequence. 
They were inhabitants of the coasts ; and as, in general, more 
littoral than deep-sea formations are accessible to examination, 
it cannot surprise us if the Calcispongiaj occur in more nume¬ 
rous localities and on more horizons than the Ilexactinellidse 
and Lithistidae, which belong to deep-sea deposits. 
From a phylogenetic point of view the Pharetrones may be 
regarded as the forms from which the existing Leucones and 
Ascones have been developod. If we assume, with Hackel, 
as the stem-form of the Calcispongise (leaving out of considera¬ 
tion the problematic preceding and unpreservable embryonic 
types) an adherent Olynthus furnished with osculum and 
stomachal cavity, we must imagine this provided with bacillar 
spicules; for not only are the uniaxial skeletal structures the 
first to make their appearance in the larva} of the existing Calci¬ 
spongise, and consequently to be regarded as genetically the 
oldest, but in the most ancient Pharetrones they exclusively 
compose the skeleton. 
Any detailed discussion of the genetic connexion of the 
genera of Pharetrones, such as Hackel has given to the genera 
of the three living families, must be abandoned in the face of 
our still imperfect knowledge of the details of the skeletal 
constitution ; it could only lead to unreliable results. For 
